Another info from Switzerland:
When I bought a new Lancia Delta 1500 in 1983 at Auto Sudan in Zug, they had in their showroom a new 037 Stradale, red of course, for sale for 35.000 CHF.
Problem was that the car was not complying with the new applicable regulations, especially exhaust without catalytic converter, so could not be road registered in Switzerland.
Only way to run it in Switzerland was on trade plates (still is the case today).
If I remember well, the Delta price at the time was 11.000 CHF.
Maybe you can find a currency converter CHF/GBP for 1983, then you would have an idea of UK price range.
Or use UK Delta 1500 price of 1983, multiplied by the price ratio 3.5/1.1