For UPVC fascia boards, my first port of call would be Wickes. As you probably know, they are widely used by tradespeople to source their materials and for bulk/sheet/strip materials I find them quite good (in terms of range) and the quality is fine.
A quick look at their website reveals they sell fascia boards in 9mm and 18mm width and my guess is your previous operative used the thin stuff. That doesn't sound enough to me to mount guttering onto.
Guttering is widely available from plumbers' merchants and I prefer Osma, which is made by Wavin down your way in Chippenham. It is widely available. Marley and Hunter are other well-known and reputable brands, all with a wide range of colours and fixing options.
You don't need a 'specialist' to do this job - it is bog-standard maintenance and fitting work, though the replacement double-glazing crowd might have you believe otherwise, as it has become a sales extension opportunity on their staple diet of expensive DG replacement.