In reply to colina:
1. Pay off any high interest debt (credit cards, loans)
2. Consider (really hard) how much flexibility you need. What are the odds of losing your income? What are the odds of needing the money unexpectedly for whatever reason?
3. If you don't need flexibility, pay down mortgage (but leave a residual £50 or so for credit rating purposes).
4. If you do need flexibility, look at remortgaging to an offset deal (e.g. HSBC 3.49% fee free). This is a straightforward comparison of costs versus where you are now.
NB. Offset deals generally not best idea as the rate is slightly higher than you might get elsewhre and applies to a much larger sum than the notional savings. However the market is doing funny things at the moment and being a saver/investor isn't a happy place on the whole. FWIW I've decided to become a net borrower again (after 30 years of no mortgage), but done so in a way that allows me access to the capital.
Usual disclaimers apply: This is not advice, merely observation; you don't know me; I'm not an IFA; you should see an IFA; shares/rates and toilet seats can go down as well as up; you may lose your children if you do stupid things, or even things that seem stupid only in retrospect; my view of risk may not match yours; do your own research; etc. etc.