The other option you could go for if you want some additional marshall style tone on a low budget is to go for a Behinger VTone thing which is about 35 quid.
It seems like so many Behinger items to bear a remarkable similarity to a Tech 21 design, and is an analogue modeller
Set your amp on clean, your bass, mid and treble to the centre notch position and off you go.
If you want cheaper and you have some soldering experience try the designs here
http://www.runoffgroove.com/The Eighteen, Supereaux, and Uno are nice and the Thunder Chief is supposed to be fun too.
Rob...