GWROB and IHT - Tenants in common

V2001

Anything more than 50% given away will be seen as a gwr. 
Expenses need to be share equally to, or your parents doing the gifting can pay more or all of them without falling foul.

If you ever move out it will become a gwr if they continue living there

Hmrc won't ask for 30 years of records, they will however require evidence of sharing arrangements for certain years they pick or seem deemed to look into.  You paying more of the bills like 90% will be a gwr. The rules are strict and you can't fall foul of them.