> Sometimes suggested its a blue merle trait?!
More like a trait of a dog who hasn't been properly socialised as a pup, and I'd bet this is the case here - sadly very common with collies, people buy them expecting a lively but relatively easy dog because they are so trainable and biddable, but they don't research properly and know nothing of the sensitivity of the breed and their pitfalls. Reactivity is common - yes, it could perhaps show more in merles because they have been bred more in recent years due to demand for unusual colouring, so more bad breeders producing pups to sell to whoever has the money, but beyond that I wouldn't consider it a colour-related trait.
What training etc do you do at home? Training isn't just about classes - I'm talking about fun stuff, tricks, search games, that sort of thing. My dogs don't go to classes but I do stuff with them at home all the time - I have to with my barky collie - her issues (again due to absence of socialisation as a pup in her first home) are so bad that she cannot leave the house, she's had three walks so far this year and only the last one held any joy for her :-( So I have to entertain her in other ways; treat balls, kongs, playing with the hose pipe (or the shower on a manky day), tricks etc. I find tricks especially useful because by their nature they are fun - there's no pressure to reach a target so the owner tends to enjoy it more I think, which means the dog does too, and they build confidence and relieve stress very well.
How long ago did you move? A lot of collies find comfort in routine and when things change, especially with the very stressy ones, it can take a long time for them to get over it. The same is true of a big, stressful event - stress lingers, sometimes for weeks. It took Phoebe nearly a fortnight to get over one vet visit. And this happens with smaller stressors - individually they aren't much of a problem but several at once, or one after another, can cause the same problem. Try to identify any and all things that worry him - any sounds, experiences, anything at all, and eliminate them as best you can. For example - I haven't hoovered in about a year because the hoover is a big trigger for Phoebe; it's not essential to me (vinyl flooring is lovely :-D) so for her sake, I don't do it. That sort of thing - you need to reduce his stress levels as much as you can because stress inhibits learning: it will slow any progress you might make, if not block it altogether. Phoebe is with me because the traffic noise around her last home was too much for her to make any progress as long as she was there.
I'll reiterate, I think you need help with him - someone excellent at reading dogs who can see what's going on and help you go about addressing his issues. Being worse at home can be down to two very different reasons - on the one hand (or paw?), he may be feeling more confident as he is somewhere familiar, so he feels more capable of trying to deal with what he sees as a threat; on the other, that threat may be made even more threatening by virtue of being in his familiar, safe place, so he can't contain his fear any longer and starts to react. Both things could be happening at once.
Re. Wagg - I'm neutral on it in general but check the ingredients, make sure he's not on one with colorings and the like in it. The Worker has them, I'm not sure about the other varieties. They don't affect every dog but they can have a big effect on some so worth considering :-)