Open up the case and look on the power supply. Hopefully there's a label there. Packard Bell is notorious for using the absolute minimum power supply for the particular system its powering, and in a totally non-standard format so only a PB PSU will fit.
I went to the PB website, and couldn't find any model 5069, but the imedia 5096 looks like a pretty standard format ATX-style mini-tower (but I couldn't see the PSU area, so I could be wrong). So, "IF" it's a standard ATX power supply, I would suggest a 400w one from OCZ, Enermax, Fortron Source, Sparkle, Antec (the TruePower series), or a couple others. Depends on whats available in your area.
I would still check and see what brand and rating the PSU in the system is - if it's already a pretty powerful system (the 5096 is a P4@2.66ghz, so that's not a slouch), the PSU may be decent. (though I wouldn't hold my breath, I'ld still look into it.)
Since I don't know if its standard atx, maybe Crashman will. He'll probably be along shortly.
Barring that, <A HREF="http://www.formfactors.org/developer/specs/atx/atx2_1.p..." target="_new">here</A> is the paper on the ATX specification. Don't know if it'll help or not. If the manual or website has detailed pinouts of the motherboard power connector, and they match the spec, and the PSU screws are in the same spot as an ATX PSU, then its ATX. Easiest way I know of is to compare it to an extra PSU if you have one.
Yes, the brand matters. Most brands are crap and highly overstate the rating. A cheap 400w may be able to put out 400w for a day or 2. What brand and part number is it?
PS: I would expect to spend probably £30 or more for a good PSU. I live in the US, but I think that's about right after conversion ($60+ USD). There are plenty here from your area, and they may have some good sources for you.
All my PB's from PII's onward used standard boards (ATX and Micro ATX). Most used PS-III power supplies (a shorter version of the full-sized ATX power supply made to clear CD-ROM drives in reduced-depth chassis).
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>
this <A HREF="http://www.google.com/search?hl=en&q=q-tech+power+suppl..." target="_new">Google Search</A> showed up several interesting links. The top one and #4 are reviews that said it wasn't too bad, but the voltages weren't that stable (sounds like a contradiction to me...), #5 and 10 are forums - they definitely don't recommend it.
I don't know what's available in your area (I'm assuming you live in GB since you quoted pounds earlier), so I'm linking you to a couple US sites - You'll have to see if they're available, or select similar models from the quality brands available in your area.