but I did notice big light brown blotches on their body the same colour as they're little spots... not sure if that's a sign of anything? It's been on all three of them that have died.
That's the stress colouration, which is normal in a fish that is unhappy/dying.
What are the conditions that the bristlenose are kept in the shop? Ask about temperature, pH, nitrate and hardness. Were they on a soft water system? Do the LFS add salt? Many do!
Nitrate shock
can kill fish when they go from low nitrate in a store into high nitrate in a home aquarium, even if the fish in the home aquarium are fine (they have adjusted to the levels).
I suspect dodgy stock, and/or significantly different water parameters from the shop to your tank.
Norfolk tapwater will be at the very maximum legal limit (30mg/l, I think) for nitrate the UK. I recommend you adopt a biweekly water change regime of 30-40% to get your tank water to the same as that of the tapwater.
Try a bristlenose from a different supplier, one that is using the same tapwater as you are. Make sure the bristlenoses have been in the shop for at least 1 month and are feeding and not diseased.
Were you specifically feeding the bristlenose or letting it fend for itself on the algae in your tank?