Hey guys, thanks for the replies, much appreciated. Effectively, I've been doing 25 or 40 mile time trials five times per week and have plateaued out - I can see what I'm doing wrong now.
I think I'll do intervals for say the first 30 mins three times per week, with perhaps a 45 min ride home at 85%, plus a couple of longer rides on other days at more casual speed. I'll just see how it goes. Think I'll invest in a turbo trainer as well, so I can do short burst of 10 mins in the garage, then recover etc without having to go out for a ride, or for bad weather when I can't get out.