Hi,
Well despite having a clean settings file the problem happened again last night and again this morning. As this looked like a bouquet issue I deleted all the files related to ABM or bouquets as well as the setting file in the Enigma2 folder. I then ran through the setup without reverting to any backup file, with a fresh scan. I then went into OpenWebif and re-organised the channels. I noticed that there was now no longer a "All Channels" bouquet, so just moved the required channels from the "Last Scan" to the "Favourites (TV)" one. This seems to have done the trick and so far no problems.
So it looks like the issue may have been waiting to trip me up, with a (probable) mistake in the bouquet setup when I first setup the box. Thanks for your help as I've learned a lot already through this issue.
Cheers