Yes a new boiler that replaces an old boiler would just be maintenance to the existing mandatory facilities rather than an improvement, so would be an allowable expense.

PS - Ensure you have the boiler safety checked by authorised personel and obtain a landlord safety certificate for your and the residents protection, you do need this to satisfy legal requirements.

This article courtesy of the Directgov website lists deductible expenses.

http://www.direct.gov.uk/en/moneytaxandbenefits/taxes/taxonpropertyandrentalincome/dg_10014027

You would pay tax on the balance at your usual rate of income tax.