Just a quick update,
I thought the problem had gone away as the unit ran for several days trouble free, but went out again today and in the 'real world' the GPS switched off again.
I also noticed that my screen kept dissappearing but pushing any key brought it back - wonder if this is some new economy mode rather than just switching the backlight off.
I'll try Raddad637's suggestion but at the end of the day, we shouldn't be having to work around this - Garmin should be looking at this. I for one will be sending an email to support.
Edit: I had a look at the suggestion and just thought I should add that my display Data Fields was set to '0' when this problem shows itself.
Edit 2 - Seems Garmin may have done a small update, but my software revision still shows 4.50 - however my battery type did change back to default Alkaline.