mark willis wrote:My question is if I buy the house for cash, if I then then re-mortgage it for say 60% of the value to get my cash back, can I then offset the interest on the mortgage against the rental income?
Yes.
See HMRC Manual BIM 45700 for explanation of how you are free to withdraw your owner's capital and replacing it by a loan on which interest is wholly and exclusively for financing your rental business and so is deductible from rental income.
http://www.hmrc.gov.uk/manuals/bimmanual/bim45700.htm