I live on a council estate and there is a large grass area with a pavement running all the way around.
Is it illegal to park my car on the grass area as there are never any spaces on the road.
I think you will find it's an offence to cross the pavement unless there is a designated crossover-point ... like a dropped kerb area ... if you bump-up the kerb, it's an offence
If you are unlucky the owner of the land could have you arrested and prosecuted for criminal damage. Plus you could be preventing the owner from maintaing the land the could then tow you off andsend you a bill !
Even if it isn't illegal, you can be certain that your local council will not pass up the opportunity to fine you for some offence thay have just thought of and then declare, with a straight face, that it has "nothing to do with revenue raising."
Go to your local council offices and find out who actually owns the land and whether it has been designated as 'common land' or not. Councils often 'believe' that they own land when they in fact do not, but a search through their records should sort it out. If it is common land then you do have the right to park a vehicle on it, but you might not have the right to cross the pavement to get there.