Any ideas why my fuel consumption would increase by about 10%
after having an FSE pump fitted ? The regulator wasn't changed,
however the exhaust pops more when backing off the throttle and the car seems subjectively more 'eager' ()

I thought the standard regulator would allow no more fuel in than before ?

I know the cold weather makes it increase slightly, but last year I was getting 26mpg (admittedly with duff MAF and lambda) during Dec and Jan.

I am, however, happier at the performance, and the knowledge that the car is less likely to be running lean is comforting

 

The FSE Walbro 341/342 pump can overcome the standard regulator.
Your description indicates a rich mixture.
Get someone that knows what they are doing to check it out but it is quite probable that, with the new pump you are running rich but there are other things that could affect your fuel consumption so it needs checking out.
Consider an adjustable fuel pressure regulator and remap.

 

Thanks for the info. I'll get the regulator done when I get
an equal length header, along with a remap.