Yep. You and everyone else want one. *sigh*. Now, here's the truth. Won't be what you want to hear, sorry:
1. You can find (current) LCD displays around 20" with a 4:3 aspect ratio. Unfortunately, you won't want to purchase them. Trust me. They are not made for use as computer monitors. If you search deep enough you'll find them...they are mostly low rez displays used for video monitoring. 
Here's an example: http://www.totalbarcode.com …umber/20RTC.html?src=TBAM
Oh, you can find some high rez ones, but they are SUPER expensive and made for pro video crap. Sigh.
2. Look for older 16:10 displays. Unfortunate because most of the ones you will find are crappy TN panels. TN really has came a long ways.... :\ I suggest not doing this.
3. If you MUST have 4:3 ratio on an LCD: Find a used 17" PVA panel. I have three of these taken from HP photo kiosks from d..err..a drug store that does photo that we service, ahem. PSU in them dies and they are soooo easy to fix, even my horrible soldering skills suffices, lol. You can find decent 17" panels for super cheap (<$50...or perhaps free...talk to local places that go through that stuff). They make great second monitors for web surfing, that's what I use mine for.
4. If you MUST have 4:3 but are more open to options: find a used CRT. You're in the UK so I dunno what it's like there, but over here you can find 21" *nice* crt monitors for <$20 from surplus stores, state surplus, hospitals, etc... Still...heavy and take a lot of desk space. I gave the ones I had away, sigh. Miss that 23" trinitron... 
5. Give in and buy a cheap 21 or 22 or 23" LCD that has a swivel and use it in portrait mode. I'd suggest this option. Wait for a special and buy two of them if you must.
Your question ("where can I find a good 4:3 monitor") is asked over and over again on computer forums all over. Don't fret, it's a *good* question. Just with a sad answer.
Even 16:10 displays are becoming super hard to find.
((((( Trust me if anyone came out with a new 4:3 monitor in a 23-27" size for <$500 that was decent, they would be out of stock in minutes.
(( Sorry for long reply, just 16:9 computer monitors are pet peave of mine--but what can you do when nothing else is made anymore. 