Okay, so...this I don't think this is going to paint me in a good light but I'll do my best.

I've been married for coming up to 12 years and I absolutely love my wife, please don't doubt that for a minute. We've known each other since we were kids and there's nobody I would rather be with.

Now, this is where it gets a little difficult, she's on the larger side...a 16/18 UK size. Don't get me wrong, I have zero issues with this. It never even comes to mind really but recently she's put on a little more weight than normal, which she is aware of and is definitely unhappy about.
I encourage her when she brings up trying to do something about it but I never broach the subject myself, she is very sensitive about it and the last thing she needs is me reminding her of it.
So far, so good...no issues there.

The problem, if you can call it that, is a bedroom one. A cliché I know but what can ya do.

Of late her weight has made it a little difficult to...well...you know. It's not an appeal thing, I find her insanely attractive...always have, always will, but of late her weight has made various positions somewhat uncomfortable and some just downright impossible (including one or two of her favourites). I feel like I am letting her down a bit in this respect but I do my best. I'm not sure how much detail I can go into on here but I'll dance around the topic like a kitten on stilts following a laser pointer.

I want to tell her about this but on the same not I don't want her to feel like I'm launching some kind of personal attack.  I'm fairly certain that she is aware of this too and we've known each other long enough to trust each other with the subject. I'm just not sure how to bring the matter up.  It's not just the sexual elements, there's health matters too but we've discussed that.

I just don't want her to think I am put off by her, I'm not. What do I do?

Thank you.

as it happens this might not be an issue anymore, last night she signed up to Weightwatchers or whatever the equivalent is...I was listening I promise...so it might just be the universe answering my question as well.

For now I'll put this in my back pocket, I'll encourage her and support her and if she starts to falter or lose focus I'll gently bring it up in a frank and honest manner as you suggest. Not to hurt her of course, I'd never do that, but to give her little nudge back on track.

Good on her for taking action.  You should too.  Work out with her, eat healthy together, etc... try to both get into better shape than you ever have been in.  If you take it seriously, she's less likely to fall off the wagon and the easier it will be to make it a lifestyle change than just a diet.