I have worked the system!
I found a site
www.navi-technology.co.ukSearch Garmin Montana (nothing else brings up the required page)
Montana 600, with OS maps bundled for £299.
Printed the page and toddled off to my local Go Outdoors
They happily gave me the Price Guarantee - £299 less 10%, so £269
The OS maps, as expected, were 2011, but they are good enough for me
There was an offer on for £50 off 1:25k OS mapping - The Manager was "dubious" about giving me the Price Guarantee because, having the offer, the deal was different to the navi-technology one. But he gave me the Guarantee with no hassle.
I wasn't bothered about 1:25k, but I was disappointed that the £50 off offer ended in April 2015!! Won't be having those then.
Unit is charging now, then I'll have a play with it when I get a chance.
Mike