Speakers are equidistant. I've no real problems, I think it's a naturally very good room acoustically due to the sloped ceiling, the alcove and the L shape. There is zero echo from a handclap. I'm seated well away from the rear wall. So it's more an idea of making final improvements. For that reason, I don't want to expend the time (which is a rare commodity at the moment) on a DIY bass trap which at best is likely to absorb upper bass frequencies. I've never been entirely happy with the rear panel and I now have an image I want up in my house anyway, so I'm thinking I may as well spend a few extra quid for an acoustic panel as an ordinary one. The bass traps are probably an unnecessary luxury.
Anyway, thanks for the good luck wishes and I'll be sure to post if I make any major changes.