to help you, here's step by step:
1) calculate the value of the next 66 years rent (i.e. Year's Purchase 66yrs at 6% of £50pa):
=(1-(1/(106%)^66))/6% x £50
= £815
2) Calculate the value of the land at the end of the current lease
Rough and ready, say 40% of today's value if the house and land were sold together, say £250k, but you use your own number.
= £100,000
3) What is the present value of that £100,000?
PV @ 6% for 66 years =(1/(106%)^66)
X £100,000 = £2137
total value of freehold =
£815 + £2317 = c£3k.
That will move a round depending on the yield you use and the value of the land.
Yield is an approximation. You can have a go at guessing a property price, but the 40% is a wild guess.
This is just for methodology DONT USE IT AS ADVICE!