www.gpstraining.co.uk/reviews/what-is-the-best-gps-navigational-device-for-walking-and-cycling-in-uk
Hopefully this site might help Stuart.
You will have two key choices. SatMap or Garmin. Both have their attributes and you will read / hear strong arguments for both devices. I'm a Garmin owner. Fully waterproof and very robust. It doesn't have the wonderful screen that SatMap has, but on the basis that four SatMap owners I know have all had issues with waterproofness / buttons falling off then if you need a robust unit go for Garmin.
SatMap's screen is second to none and also has the advantage of regional maps so mapping can be cheaper.
Happy to help you navigate the menu of Garmin if you decide to go down that route. I've been a user for 15 years and can't fault them at all.
Regards,
D