Insurance question!?

It's very annoying I know.
I had something similar happen to me years ago when some tw*t suffed a GTi into the rear of my parked s1 Daimler Sovereign and drove off, when I eventually found the culprit, I lost a load of money on the settlement because I didn't know as much about it then as I do now.
The woman you spoke with will be going from book prices or will have pulled up some current ads for similar age Novas online, (she will have selected the lowest priced ads).
It's a sort of, (loaded), negotiation process which you can ultimately take to the Insurance Ombudsman if you're dissatisfied.
It's up to you to search out like for like ads & select the higher value ones.
awoo's point about sorting it out directly with your neighbour is a valid one. She will, presumably, be liable for a couple of hundred pounds of insurance excess plus they'll load her insurance premium for a few years because of this claim. If you can get a more reasonable quote for this repair work she might perhaps be persuaded to finance the repairs to avoid submitting a claim?
If this isn't the way forward for her & her insurers are being intransigent, perhaps ask your insurance company if they can become involved & mediate.
Failing this, stick to your guns, submit evidence that the car is of a higher value than their assessed value via current ads, work your way through their complaints process & escalate to the Insurance Ombudsman. I think you'll get a resolution before you take it that far.
Financial Ombudsman Service (includes Insurance)

And remember ....

Of course, if she's got expensive work to be done on her own car it's not going to be worth her while sorting out with cash. The fact that she's directing you back to her Insurer probably means that they have warned her not to have any dialogue with you on this subject, (which is quite usual - the threat is usually that you risk invalidating your insurance policy if you don't comply).

It is possible that if everything goes your way with this that they might pay for the repairs rather than writing the car off, (if that's something you'd want?)

As far as her Insurers are concerned, I should try to keep everything in writing / email. Claims adjusters are trained in the art of stiffing people & have many sneaky tricks in their repertoire to railroad you into accepting a fraction of what you're due. The person you're speaking to is probably getting a bonus on a sliding scale, i.e. the more they stiff you for, the bigger their next pay-packet.
Check first if your own Insurer is able & prepared to intervene or even if they can offer advice.
If not start to amass evidence to back up your perceived value of the car. Check all relevant sites, ebay, Gumtree, Car & Classic, Preloved, etc. pick out cars which come within a year or two of yours, are in similar condition, have same engine & same number of doors, have a similar mileage, etc. and print out 5 or 10 of the highest priced cars, (copy web addresses of ads in case you start to correspond by email).
If you go into W.H. Smiths or any big newsagent you might be lucky enough to find a 2016 classic / collectors car price guide still on the shelves, (which might cost you £5-£15.00). If there is a choice of more than one, pick up the copy which gives the best price. Copy the front cover & the relevant page. Highlight your car and the price which you believe applies to it with highlighter or ring with red ink. Keep the newsagent receipt for the price guide, you can ask for this back later, (although asking doesn't mean getting).
If you find the 2016 guides are sold out in the newsagents, give a shout out on this site in case anyone has a current price guide, you're just looking for them to email you a scan of the front cover & the page which relates to your specific model.
Your neighbour's Insurer, (if they haven't done so already), will probably send someone out to inspect your car. They'll give it a good going over & take loads of photos. The goal is to find as many faults as possible to batter you down on any settlement. They'll even go as far as measuring the tread on your tyres & taking money off if your tyres were approaching the end of their lives. With my Daimler I'd have described it as a condition 1 car but the Insurance adjuster was portraying it as a condition 3 junker. It's in your interest to make sure that your car looks as good as possible and, if possible, disguise any obvious flaws.
Also, pick your model out of this list & you can highlight the rarity of your car by accurately detailing how few are left on the road in the UK.
Howmanyleft : Vauxhall Nova
Finally the fact that it's an accident damaged repaired write-off. Yours will be a Cat C / Cat D write off. I don't know how much this impacts value & there is a lot of conflicting information online varying between 10% to 50% off the value. The amount that seems most sensible is 70% of market value of its unrepaired equivalent. Again, perhaps your Insurer can offer advice, ask if there is a formula they apply?
I have a friend who used to work in Insurance, I'll ask him if he has any knowledge of formulae, etc. which are applied within the industry & feed back.
So if, for the sake of argument, you produced 10 ads & a current Official Price Guide valuation showing your car is worth £2K then argue that the 'write-off repaired' status means it's worth £1,400.00. If the values are quite high and it's worth more than this you might be on the borderline as to whether you're compensated or whether they'll repair the car. 
You're going to come out of this with more than the couple of hundred quid they've tried to fob you off with but it's going to take a bit of work & patience on your part.
This is likely to drag on for quite a while but don't despair!
The Insurers want you to despair, they want you to get fed up of the time it takes to resolve and beg them for that £200.00 cheque!
Don't give them the satisfaction!

I was in W. H. Smiths, they had 2016 price guides in, I checked up for you.
Even though the guide covers the appropriate years, for some reason they don't list the Nova or its Opel counterpart!?