Thanks for your answers - I'm still unsure of the answer though

So, sounds like the answer is definitely one of the following:
1) 100k - original cost of property when it enterred the "rental business".
2) Up to 150k - the "value" of the property when it was first let (although it would be interest on 135k, the size of my loans).
3) 125k - original cost of property plus subsequent capital refurbishment costs.
I have evidence to back up the valuation of "150k", 4 estate agent valuation letters and a surveyors valuation as used for the remortgage. Although if the answer is 1 or 2 then I guess this isn't relevant.
I've trawled through lots of information on the internet and read quite a few examples, but nothing I've found so far deals with this scenario exactly.
On a general level, is the interest on loans to make capital improvements an allowable expense against your rental income?
Cheers