Went for one of these -
https://www.ebay.co.uk/itm/Campbell-Hausfeld-PL1558-Air-Powered-Caulk-Gun-New-/132754467991
I've paid all of £14 for the tool, and about £22 for shipping...
So about £20 more than a Clarke one, but this one depressurised on trigger release. Overrun has always irritated me with manual guns, and I recon this one will be a load nicer to use.
One thing worth noting is it may have a 1/4 NPT thread on it. All Snap On and a lot of Chicago Pneumatic stuff has the same but everyone just fits a 1/4 BSP fitting.