On the issue of letting surcharges on mortgages, we were advised by a particularly good broker that we have used for the last 5 properties/mortgages to advise the lender from the outset that, because of my employment, we would be required to move on a regular basis and would therefore let the property. Our current lender got rather shirty about 18 months ago when we went abroad and proposed a 1.5% surcharge, plus a whopping admin fee to make our family home a 'buy to let' property. After a fairly long battle, the company agreed that we had disclosed our intentions when they agreed to mortgage our property so they waived the surcharge, waived the fee and paid us GBP500 to boot!
They also agreed to have the property independently valued (gratis) as it is a large Victorian property sandwiched between some much older terraced cottages in a National Park. When we considered a further advance, they applied the usual RICS formula and stated that the Loan to Value ratio was too high - based on an assumed market value distorted by the small terraces nearby. I protested and they agreed that they had made a mistake and hence a free GBP500 valuation. Result!
Bottom line, keep meticulous records of all expenditure (hard copy and electronic - scan all receipts as most seem to fade nowadays) and keep safe all correspondence with HMRC, your bank and mortgage lender - plus notes of all telephone calls and discussions - and get names!). A simple yellow treasury tag and a hole punch(raid the back of the Sqn stationery cupboard) and file everything sequentially. It builds a narrative! Be a good landlord - our property in WB was on the corporate let market for a number of years and although it was being professionally looked after I did pop in from time to time when I was in the area. It was nice to meet the tenants (and they made sure the house was spic and span knowing that I was visiting), and generally there were a few niggles that needed simple attention.