Thanks all for your comments and sympathy.
Yes, it's a nasty feeling, and it's the first bike I've ever had stolen despite commuting by bike since I was 7!! Clocking into my early fifties now.
Frame number was: CL580L1814
I had it registered on Bike Register, so I've updated it's status to Stolen.
I've also posted about it on the Bristol Cyclists Facebook group, and told various other people in Bristol in the cycling community to look out for it.
Yes, it was locked to a Sheffield stand in a fairly busy public place, during the day etc, so maybe carefully targeted.