<Contributed Amount> / <Original House Price> = X / 100
Say the house was worth £200,000 initially.
£36,150 / £200,000 = X / 100
X = (36,150 / 200,000) * 100 = 18.075% investment
Now, figure the current price and how much they should get back using the same type of formula.
<Return Amount> / <Current House Price> = <percentage invested> / 100
or from the above example, assume the house is worth £300,000 today :
<Return Amount> / £300,000 = 18.075 / 100
<Return Amount> = (18.075 * 300,000) / 100 = £54,225
This only takes in the values you put into it. In your case, your initial house price was the total you put down and your parents put down (£82,500 + £36,150 = £118,650). The current house price is the amount that you'll be getting back AFTER the remaining mortgage is paid off. If say your house is worth £500,000, but you still owe £350,000 mortgage, you'll only be getting £150,000 back and thus that is the price listed as <Current House Price> not the street value.
This assumes that you paid the mortgage the whole time without any help. If they contributed to the monthly mortgage, you'd have to figure out how much of that they invested in.
Let me know if there is anything more you could use help with there. It isn't that hard, unless there are other things that come up. I'd recommend being completely honest with them and showing them all the paperwork; don't want a few dollars hurting family relationships.