For this length of time, you need to be looking at stockmarket investments.
As a first dip into it, I would suggest getting yourself the cheapest FTSE tracker you can find. As they are just trackers they are much cheaper than managed funds, so your investment will not be eaten away by overpaid fund managers.
Also, the monthly deposit will even out the bumps in the market.
For this amount of money, you can look at an ISA to avoid CGT in future. Something like the Legal and General UK Index ISA.
Hope this helps...