Letter from in-laws as proof of suitable living accomodation

Hi there,
Quick question about this. UKC wife and USC husband (me) are shortly moving back to the UK from the US with our UKC/USC daughter. We are intending to stay with my wife's parents for a time while we look for somewhere to rent. My understanding is that I must include a letter from them in my supporting documents as proof of suitable accomodation. The house is more than adequate, and they've e-mailed me a letter with the number of occupants, rooms, etc. My questions are:
1. Does this need to be an "official" letter? I've got an e-mailed copy. Do I need a signed original? Does it need to be signed in front of witnesses or anything?
2. They own the property, and don't have a mortgage. Do I need to provide proof of this? What is acceptable proof? I've heard I can get this from the land registry website?

Many thanks in advance,
Nate

1. Email may suffice for a US application. However I would still get an original letter addressed from them to the ECO at the British Consulate describing the residence, how many people currently live in the residence, how much if any you are to be paying in rent & utilities, etc... They can also state that they have no mortgage on the residence. The letter does not need to be notarised or witnessed.

2. As far as documentary proof you need a copy of the deed if they have no mortgage. If they are unable or unwilling you can print a copy of the property details from the Land Registry Website if it is listed there for a small fee of £4.