Hey
As someone in a similar situation (though I invest much less every month, but every now and then have a couple thousand spare to throw in)... I've been investing since my early 20s, and this is what I did:
I started off with a £1000 investment in Xstrata. Knew very little about them at the time, but it was recommended to me. I played with a virtual portfolio for a bit, too, looking at high yielding stocks and the big name players - groups like tescos, RBS, etc.
For a long time my portfolio was very flat, over the last two years (like many I guess), I've been very flat. The last 6-9 months, though, it's been a killing, throwing a 40% profit or so.
I did what many would advise at first, I had quite a diverse portfolio in a range of sectors (Tescos, ARM, Xta, ICAP, amongst others)... This was very flat profit, and my main reward was from dividends.
In the last 6 months or so, now that I feel more confident, I've broken my diversification to much smaller groups. I went with ARM, XTA, and SVT for a while, then sold out of SVT and moved that into XTA. My portfolio sits at 65% in XTA, 35% in ARM, and I've ridden one hell of a boost of late.
Too much diversification will kill you, as the 15£ commission I currently pay through rbs-sharedealing is quite alot. Certainly the only time I've gone in on speculative trades there was BP after the oil leak and Standard Chartered after their scandal recently. They paid off.
Stay confident, play the long game on shares, and dont sweat it when things dont go so well. If you can reason with yourself well enough to buy them as a long term plan originally, dont break the faith until you see news that actually changes that - ride market fluctuations and unless you have to, dont look to cash out unless you're no longer convinced of long term viability
I'm sure I'll get slaughtered for most of what I've said there, but like I said, it's worked for me, last few months have been a 30-40% skyrocket in portfolio value.