Two points --good camera -- £300 is too much!
Although the SD5 cost well over £500 when new, it is now two generations adrift, and things have moved on, with prices falling.
I've had my SD5 for 18 months, and have been very pleased with it, but it does have a couple of minor things that you need to be aware of:
The battery is internal, and
AFAIK, only the Panasonic VW VBG 130 battery will fit, and they are quite expensive (about £60 from Jessops, for example.).
The later models, like the SD9 and later models, had external battery locations, and some of the cheaper 'clone' batteries can be made to work - but not on the SD5
Second thing to check. The SD5 has an internal fan for cooling. Make sure it is not too noisy, or it will be picked up by the camera microphone. Should be OK - mine is not too bad.
Later models do not have a fan.
Also the SD5 won't record progressive footage like the later models. That's not usually a problem, but you need to be aware of it.
So - a good camera overall, but not worth more than about £200 second hand (IMHO of course!)