Getting my flat ready to let out . . . . what can I claim back tax-wise ?

I assume you have taken financial advice from an IFA before deciding on this investment.

Tax advice should be obtained from a qualified tax adviser, but in order to frame your questions I suggest that you read HMRCs Property Income Manual
http://www.hmrc.gov.uk/manuals/pimmanual/index.htm

Do bear in mind that this manual is ONLY HMRCs interpretation of the law, it is not a comprehensive tax planning manual, which is why I suggest that you seek tax planning advice.

My understanding of this and I have rental property is that any work which you do to a property before it is rented out is a capital expense and is recouped from the the capital gains tax when the property is sold but any work which you do to the property while it is being rented to repair what is already there is deemed as a tax deductible expense. Home improvements are excluded. But you can claim 10% per annum of the rental income as fair wear and teare.g. if you get £5000 per annum in rent you can claim £500 against tax for replacement furniture, appliances and carpets ...if you are quite frugal you can do a lot with this 10%. Anoter thing that you can do is to take out a mortgage or a loan for the work and then claim the interest against the profits of the rent.