Just as an update - I contacted BT Customer Services about a month ago to describe my problems and they did everything they could to get out of admitting there could be a problem with the HH5. According to them all firmware update issues had been solved, the line was showing as fine with no faults etc etc and that the problem must be with my devices that were losing the wifi connection, and I should contact those manufacturers. As usual with BT Customer service it was like banging your head against a brick wall.
I bought the TP-Link router straight afterwards and haven't had any problems since, everything working perfectly. So it wasn't the HH5 with a problem - yes of course!!
I feel like phoning BT back to tell them how wrong they were, but it will only end in more frustration.