I can't personally recommend one because I haven't got one yet but I've been looking at the Kane 504 which can be had for less than £300 inc vat. You might get a better response on the plumbers forum.
The Testo 310 is a good analyser, but full kit that includes a printer is around £500. Try the Kane models for spec and prices. Remember to add on costs for printer kit though.
Tpi are basic but ok https://www.bes.co.uk/tpi-709r-flue-gas-combustion-efficiency-analyser-kit-1-22259 If you dont need the printer they are a little cheaper If you need a record of the test,just photo the analyser's reading !