I'm sorry i don't know the legal bit. However, have you considered that trying to cling on by your fingernails in a house where you are not wanted by your own parents is a bit futile and ultimately (whether the law can help you or not) will end in tears? I wonder if a court might also consider that the money you have put in equates to around £100 a month for 4 of you? which is way lower that you would have had to pay in mortgage or rent elsewhere?
I can't really see how this is to your detriment, seeing as you have saved so much money over the years. However, as i have said that is not a legal answer.
If you have a solicitor, why aren't you asking htem?