I have a working cocker in very good health who turned 5 in July 2024. She is a pet and we intend one litter only. It would be her first mating.
Do breeders have positive experiences of mating in these circumstances? I understand that there is an increased risk of complications because of her age and that we need to work closely with our vet  but I am not able to find any information around quantifying risk?

I'd say that there are plenty of working cockers around, why risk it? If you don't actually work her then you're not adding anything to the breed, just more dogs in an already flooded market.

May i ask why you want to put her through the risks of a litter particularly as an older bitch ? If you want one for yourself it us better to speak to her breeder and see if u can get a puppy from same lines
I see you say she is very healthy which is great but have you had the correct health tests done for her breed ? Not just a vet check.
I know other experienced breeders will have advice amd questions for you

I have had a litter from a maiden bitch at 5 - all went really well.  (Different breed though) 
There are so many puppies available at the moment that its not east to find homes for them.  You need to also consider that, how would you cope if you had some left at 8 weeks +
Has she had any health tests done that are relevant to the breed?

5 is when you would be thinking of taking the last litter from a bitch that had had her successful, uneventful first litter at 2 . It’s not worth the risk

Totally agree with the 'whys' and 'why risk its' here.   We pretty much retired our bitches by that age, having taken just one, or maybe two litters, for our next generation.

I've been looking at the other 'well known' puppy sale website and in my breed at least, many people are reducing their asking price which, even if this is the holiday season, is a serious indication of what's going on in the puppy market at the moment.

Please abandon this idea and just love your bitch without putting her through the very real risk of mating/whelping and rearing a litter.   Better yet, have her spayed.

Assuming that she has been hip scored and DNA tested for Acral mutilation syndrome (AMS), Familial nephropathy (FN), and Progressive retinal atrophy (prcd-PRA), (and the stud dog the same) and you have a waiting list of people who are genuinely interested (not just friends saying "Oh I'd love one of her puppies" because they melt away like snow in summer as soon as the puppies are actually on the ground!) then it'd be worth considering. Working cockers are very plentiful and can make challenging pets (even more so when crossed with another breed) so good homes aren't easy to find. So think long and hard because being morally responsible for the well-being of 8 more dogs for their whole lives is quite daunting.

I have seen a lot of puppies advertised lately that haven't managed to find homes at 12 weeks +.   Its really hard to find 6-10 super homes for puppies .  It wouldn't be so bad if dogs only had 2 or 3 puppies but it would terrify me if had whole litter on ground and couldn't find those good homes. and keeping them all is NOT an option.   I've seen rescues taking in whole litters that failed to sell.
if you want another one go back to her breeder and put your name down for a related puppy.  its the advantage of pedigree dogs, you can easily find a relation of the beloved dog you already have.  easier, cheaper and more choice of colour, sex etc