Just one thing on your mortgage change - why are you changing to a 10-year mortgage? Personally, I'd leave it at 15 years and pay it off as if it's a 10-year mortgage.
For the interest rates that you quote (5% pre-DIRT and 3.33% no-DIRT), you'll be getting a better rate if you put against your mortgage (prob 4.9% or thereabouts, depending on what your NIB rate is). Having said that, there are many other savings accounts where you can get a much better rate than 5% so you may be able to earn more on deposit (post DIRT) than you save by paying off your mortgage. Have a look at the best buys section. Also, there are loads of threads here covering the "I have a lump sum, should I pay off my mortgage?" which have other good info.
Personally, I'd either split between the two (mortgage vs savings) or (more honestly), I'd save half in a high interest regular saver and buy some equities, considering the prices at the moment - I'd do this with a short-medium view (1-5 years). I'd pay off my (hypothetical) 15-year mortgage as if it was a 10-year one and throw more income against that on a monthly basis. I'm no expert mind you - that's just what I'd do.
Sprite