@Sheena13 Back in the olden days, electric blankets were associated with a lot of fires and injuries. There have been a lot of technological improvements since then, so younger folks might not be so wary of them, but they're still not the norm in warmer countries and those with better insulation and central heating.
So I read your post, and I was thinking, maybe if you're offering a budget private room and competing with hostel dorms, an electric blanket might even be enough! After all, it's Scotland, not Bali. But then I looked at your listing (which looks like a really sweet place) and thought, that's not really your market. With an entire cottage, at an off-season rate comparable to a hotel, your guests are generally people who are on holiday and don't want to have to think about their host's utility bills - they just want to be as comfortable and "cozy" (to quote your listing title) as they can get.
Intuitively they know they're paying more per night than a long-term residential tenant would be, so they expect a greater degree of creature comfort in exchange for that. They might appreciate an electric blanket, but they won't want to be told not to use other heating implements to make the cottage feel as nice and toasty as it can, because they're visiting to have fun and not just muddle on through like the locals.
Unfortunately, a lot of people have a daft sense of thermodynamics. Just like the ones who want to keep the windows open all day in a 39 degree heat wave, you'll get those who keep heaters on all day while they're out and then open the windows for "fresh air" when they get home from their daytrip. My suggestion to this would be to give clear instructions at check-in about how to maximize the impact of the heating systems, and offer the electric blanket (placed visibly near the bed) as a supplement. But also, to maintain nightly prices that make it possible to absorb the cost of guests who ignore your advisory altogether. I'd also suggest emphasizing environmental concerns about fuel usage, if only to drive home the fact that this is not just about your personal costs but also the planet's.