Hi Sue, that's great news from your lender! The good thing is that you are thinking positively. You are doing the right thing in viewing your property as a 'home' first (intending staying 10/15yrs) and as an 'investment' secondary. Your homes' value is going to drop even further in the next 12 to 24 months and maybe even possibly longer. In that time you will see and read some horrendous stories with some people losing their jobs, national repossession figures going up; you know, the usual doom and gloom. 10 to 15 years is a relatively long time and these things are ALWAYS cyclical. In 10 to 15 years your home WILL have clawed back all of the negative equity and WILL show you a profit in financial terms eventually.
The Libor rate is very important for banks as it is the rate that they use to raise money on the money markets. There is a difference between the Libor rate (amount THEY are being charged) to what they will charge to their customers with mortgages, loans, overdrafts etc. This difference in the 'Libor' rate and the interest rate they charge their customers is the 'main' way that they make their money! Depending on a raft of complicated banking criteria such as money flows, profit projections (or debt), shareholder dividend payments and a hundred other factors totally out of yours or anyones control, as to the future increases/decreases in the Libor.
The sooner that you personally can get a 'fixed rate' on your mortgage that you are happy with and that you can afford now, then please take it! If you want some kind of peace of mind and to be able to sleep at night, these are very unpredictable financial times and NO ONE knows with any certainty exactly what is around the corner.
Good luck.