Is it the speakers that are sh*te, the headunit or both? Ideally I will replace all but which should be first for best improvements?

I would always start with the headunit. Fitting better speakers will make a difference, but they usually require more power - and you need to spend a reasonable amount of money for a decent speaker upgrade. I dont know the power output of your headunit, but even with factory speakers, an increase in power will make an improvement. The order would be then:
1. New headunit
2. Sound deaden the doors
3. Replace speakers

Others may disagree - but then, its a bit like "should I increase the processor speed on my Pc, or buy more RAM?"

Good Luck

Head unit is very poor, despite having a Kenwood label on the bottom of it.

Replace that, then look at front speakers. Sound deadening has an excellent effect in the scoob as mentioned above.

Ok, headunit first it is then!
Just need to get hold of a blanking plate as I'll be sticking in a single din Kenwood.

A pocket instead of a blanking plate is a common way of filling the second hole and can be had from a dealer, part number 66128AE000 - looks like this

http://members.lycos.co.uk/jockytiktok/DSC0020.jpg

Steve

Looks tidy, thanks m8

Or you can have a flappy thing which is a tenner plus vat... have a search on single double din