What's a good affordable 9mm semi auto pistol?

Your lightly used Glock 17 and MP9 are good choices.

I would recommend Ruger SR9

http://www.cheaperthandirt.com/product/52387

Basically they looked at what the Glock did well and copied it, with minor tweaks:
-significantly thinner in the grip. This is the Glock's one real flaw...the grip is too thick for most people so they shift it in their hands a very small amount throwing their aim off, and they often don't even notice. (This was also a problem with Ruger's previous semiauto series...too thick in grip)
-ads an external fire/no fire safety switch to the side. Glock has no safety switch. You don't really need one if you are really following proper gun handling, but some people like them there.
-ads a loaded chamber indicator. Glock doesn't have this. Again, you don't really need one if you are following proper gun handling which says treat all guns as if they were loaded, but some people like this feature.

Honestly, the external safety and loaded gun chamber indicators are good for novice shooters. This is the vibe I get from you so it is why I made that recommendation

I own Glocks and I find them safe and extremely comfortable to shoot. But what someone wanting a gun needs to understand what might be comfortable for one person may not be comfortable for another.

I've probably said it 50 times on this forum, if you can't handle it safely and shoot it accurately it's a pretty useless gun. I suggest going shooting with friends that may have guns you are interested in or going to a club where you can try shooting various firearms to determine what suits you the best.

Ruger p95

My input is:

http://answers.yahoo.com/question/index?qid=20121102201731AATH8Cg

Because that's the best way for you to shop for a gun.

Sig Sauer SP2022
CZ P-07 Duty
Ruger SR-9

Springfield XD.

To answer your headline...Glock.

I have a 1911 with extended magazine that takes 25 in 45ACP
Yours for £500

Just about all the newer striker fired pistols have something in the $500-700 range

I sold my glock 17 and got a ruger SR 9..I love the SR ruger..its everything the glock isn't...lighter..easyier to grip...nicer to shoot..a as new in the box sr should cost about 400...a glock about 450/500

Both the FNP/X 9 and the CZ Phantom or CZ P07 are about 100 less than the Glock or Smith and are quite good. The XD is also a bit less. The EAA Witness steel frame guns I also like quite well, but I haven't shot their poly frame Zavasta imports.

Thinkingblade

Glock 17/19, M&P 9, Springfield XD 9.

gun shop

consider Norinco NP 34
Right - its a chinese made copy of Sig Sauer. But its a surprisingly good gun, reliable and the materials are great quality.
And its around 300 USD brand new.