Thanks again Incredulum, much appreciate your responses.
Just read http://www.hmrc.gov.uk/manuals/pimmanual/pim2505.htm which says the same as you, i.e. that the rental business began in November.
With regards it being the "Value" of the property when it is first let - that sounds great to me, is that definitely the case when it's a first property - I can't find a pim on it...
I tried to make all the numbers easier to understand before, in truth the mortgage was an offset mortgage. It was one large offset mortgage facility secured against two properties, the one I lived in and the new buy-to-let one and then following the sale of my home, the offset facility was reduced and secured just against the buy-to-let flat, (6 months ago I moved it on to a fixed rate standard buy-to-let mortage).
This means that my offset balance (size of the loan) went up and down every day, so I am trying to find out what the cut off point in the offset mortgage was for claiming the interest.
Are you aware of offset mortgages being treated any differently to other loans?
So, to summarise, you think the mortgage interest on 95k is claimable until Nov/Dec (as long as the 15k is wholly and exclusively), from Dec the interest on 150k is claimable as an allowable expense?
I've repeated the example situation below.
1) I owned and lived in a flat at the start of April 2010.
2) I buy a new vacant flat, with the intention of letting it out once refurbished, for £100,000 in May 2010 using a mortgage of 80k, I also have a separate loan of 15k.
3) I sell the flat I live in in August 2010 and rent accomodation.
4) I spend 25k on the refurbishment of the flat.
5) I complete the refurbishment of the flat in November 2010 and have it revalued at £150,000, the new mortgage is for £120,000 and I still have a loan for 15k.
6) The flat is rented out in December 2010.
Thanks again!

May I ask how come you know so much on property tax Incredulum?