Just an update on this. After struggling to split the headlight last night and attempting to clean the interior via the lamp access holes and failing miserably I left it alone and approached it fresh this morning after doing a bit more internet research. I was wary of placing the whole unit in the oven for fear of melting it so removed it completely from the bike and set to again with the heat gun.
This time I warmed the unit more evenly as it was easier to access off the bike and for much longer - at least 10 minutes going around and around to really soften the mastic. Wearing gloves I was then able to prise it apart almost straight away giving me complete access to the interior. I stripped this completely and cleaned both reflectors and the black interior surround with a damp microfibre as there was quite a film of dusty muck before polishing it up with a bit of Mr. Sheen and a dry microfibre. I kept the polish off the reflectors but cleaned both lenses with a spot of Autoglym.
The dipped beam shroud on the H1 lamp was loose and upon investigation it appeared that one of the supporting posts that it sits on had worn/broken away over time allowing it to rattle about. I removed it by carefully bending the metal tabs at the rear, cleaned it and then reset it by bending the tab over the broken section in a closer manner to tighten it all back up. These tabs are quite soft and won't stand much bending so be careful if you try the same.
After a clean the whole unit looked a lot sharper. Above the H1 lamp the reflector silvering was flaking so I removed all the loose bits before reassembly. The light flow seems to be sideways rather than above so I don't think the beam pattern will be any worse but I have yet to ride the bike in the dark so all a new experience for me.