how to flush a fuel tank out ?

Nice Lada, not seen one that rust free for many years

I normally go with using some of the old fuel (strained if it's that bad) and a handful or two of clean penny washers and lot's of shaky shaky action. Drain, add a little fresh fuel and shake then repeat till the juice runs clear. Always on bikes mind but the rules are the same, had to use some red product once to seal a ropey Kawasaki tank but can't for the life of me remember what it was called.

My dad had a light green estate same as yours he bought new on a E plate. Tree fell on it in that storm of 87, got repaired though and travelled many miles in it before he got a Sierra Laser Estate as a lease car with a new job. Wish we'd been able to stash the Riva in a garage somewhere for me to play with now.

I had very similar looking deposits in the tank (and the rest of the fuel system) of my Triumph 2.5PI. That had also stood for approximately 15 years.

I used Swarfega Driveway degreaser (cheap from Toolstation), followed by pressure washer action on all the areas I could "see" through filler and sender holes. That allowed me to get about 75% shiny. Then made up a fairly strong solution of caustic soda (you can get the crystals from DIY shops for drain cleaning - read the instructions - it's nasty!) and soaked the remaining side for about 4 hours. Then shook vigorously, rinsed, shook, rinsed etc. It's pretty clean now!
Be aware that caustic is safe on iron and steel but will eat aluminium, braze, zinc plating, copper and brass so use it carefully....

I also considered buying 5L of gunwash (cellulose) and trying to dissolve it that way but didn't actually try it.

Good luck

Nick

I can vouch for that - I used caustic to dissolve a stuck aluminium seatpost out of a steel bike frame! It ate about half of the post before I managed to move it!
Cheers
Duncan