1. As you are due to review your mortgage it makes snese to borrow the extra within a mortgage as the rates are cheaper.
2. Cashback mortgages will tie you in to a lenders standard variable rate for between 5 and 7 years, with interest rates moving upwards and likely to go higher you could regret this decision.
3. Long term fixed rates are a great idea, however you have to balance against your own lifestyle, 15 years is a long time to plan ahaead and indeed your plans can be wind when life throws you a curve ball and your circumstances change. If you are young and kids havnt happened yet etc then I would go for a shorter term, the longer rates are better for those who have been and done kids, they know that their circumstances are unlikely to change and are just doing the final run to retirement.
4. NO! It does take time to organise a mortgage and is better to have in place now, if the interest rates change again it is likely the cost of fixed rates etc will rise so is better to get in and secure a rate now. This way you wont have to pay the lenders standard variable rate as you will be able to swop immediately.
Regards
Stefan