The ultimate choice would be something like a Satmap with OS maps on, but don't want to spend loads as I'll only use it occassionally.

I'm considering a phone upgrade and Nokia maps or viewranger is an idea. Otherwise there's the iphone but how depend are they on getting a signal?

Or a cheap PDA with maps on it?

I think I'll plump for a large scale map with print outs from Streetmap of the areas where i need the detail but wanted to know what other options there are.

Cheap PDA/Windows Mobile phone & NaviMapper/NaviComputer. You download the Openstreetmaps maps using NaviMapper and transfer them to the device. It's all free and you only download the area you need to the zoom level you want.

if you are after cheap Ordnance Survey maps I would suggest getting a cheap Window based PDA and download Quo (free) and install Quo mobile onto the device

http://forum.mapyx.com/Quick_Guides/Quo_Mobile/Quo_Mobile_Quick_Guide.pdf

follow this guide.

they have 1:25k maps at 1.89 (ex vat) for a 10/10km square.
The 1:50k are half that and are 40/40km I think.