Firstly, a pre-nup agreement is NOT legal in the UK, and as already mentioned if a relationship (living together as if married) lasts for a period of time, then he/she can claim a proportion of the estate. The 50/50 split thing is something you hear about in pubs and is not always the case. If someone living with you paid for double glazing to be fitted, then they would have a claim, regardless of time spent there. It isn't a matter of trust, you can trust someone now.. but can you trust them in 2 years, 5 years ???
Nobody wants to end up homeless.. myself included. I'd been paying the mortgage for about 19 years before my 2nd marriage, and continue to pay it solely, even though I've been married just over a year now. It would not be fair for someone to marry me and profit out of it, whether done intentionaly or not.