It's a very difficult one MattW. Many say as much as you can afford, you'll get a different answer from a financial adviser as from an ordinary joe on the street (my FA is obsessed with me providing for my future - savings I've been making for 10 years which were earmarked for house improvements he's wanting me to syphon away for retirement).
As a example my pension scheme gives possible examples:
Male aged 30 now retiring at aged 65 if you contribute £28 per month you will receive £144 per month gross on retirement (£5K per year). £55 per month brings you up to £10K per year. Bear in mind that if you are in a company pension (as this is) that your company will also contribute so if they contribute equal to you if you pay in £55 per month they will do the same - usually done as a percentage of your pay.
These things vary greatly so do see an adviser or the pensions coordinator at work. Bear in mind like my FA they will urge you to save, save, save so be firm on how much you need to have a comfortable life right now (who knows if you might die before retirement?).
Also bear in mind liklihood of inheritance, although these things cannot be relied upon you will know for one thing whether you will have nothing at all or whether you'll be retirement age yourself before your parents or relatives possibly pass away.
*It is not necessary to understand things in order to argue about them. -- Pierre De Beaumarchais