Op, there's definitely a market for dog friendly accommodation. We book at Little Dumpledale and they are often booked up a year in advance. They are not cheap, but have worked hard to establish themselves in a niche. Us doggy folk are more than happy to pay IF it really is dog friendly. By that I mean that the garden is properly and securely fenced, flooring is tiles down stairs, you can bring more than one dog of any size ( I think the record is 19) - the accommodation provides a doggy creche service, and they give advice on where's good to take dogs in the area. The cottages are also great for humans, with dishwashers, power showers, king size beds and good standards of decor/furnishings.
Washing line is handy for things like swimming costumes that you wouldn't want to tumble.
Speaking as someone without children, we deliberately holiday outside school holidays, and would be deeply depressed by a noisy brood of children staying elsewhere in the accommodation complex - if you don't have them yourself then you don't want them on your holidays. Also, child friendly accommodation requires particular standards of safety/ equipment such as cots, high chairs, fenced off pools etc - nothing wrong with being adult only. Given this is Mumsnet, you'll probably get loads of parents discussing bookings, but if you talk to other providers they'll confirm it's non parents and empty nesters who are providing your year round business, and they will appreciate comfort. Why not advertise somewhere like the RSPB magazine? Or other wildlife magazines? Wildlife/bird watchers tend to be quiet, responsible types who will appreciate good accommodation at a time of year that fits with their requirements e.g. overwintering waders, seabirds in spring, rare butterflies in summer. Given your Pembs location, you could do very well in this niche.
I'd say people's expectations of holiday cottages have risen considerably in the past few years, and what would have been OK a few years back (no TV reception, dodgy old furniture, grotty bathroom) just won't cut it any more.
Welcome pack with plum jam and home made bread - fab!