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rj-
I was hunting around to name a particular filler that works for the depth in one hit. Bonding vs multi finish is quite different, bonding is very good at building depth without compromising the structure and it still dries fast. There are some fillers like Murex but they're generally specified for outdoors because they use elements like ground marble to provide waterproof hardness.
By all means try multi finish, it takes longer to dry and tends to shrink away from the walls of the channel because it's being applied too thickly.
Airhead
It's not that difficult but you need smallish amounts of a few materials that come in larger quantities.
Hopefully your wiring is in some form of trunking possibly plastic, pinned into place so that it's easy to plaster over.
PVA the channel, mix up some bonding plaster (not multi finish), slap that in and roughly level it off, best if its a little below the surface but just less than proud is best.
Let the bonding plaster dry, 24hrs should do it if the central heating is working. Wash the bonding off the walls surrounding the channel with a sponge. Should now be clean walls.
Use some TX110 from toupret or similar if you have another filler you prefer. Fill the channel to flush. If you want a slight texture to match surrounding walls then get creative with a sponge. Let it dry a bit then sponge of any excess.
Paint with Gardz from Zinsser. Let that dry and finish with emulsion.
So you need pva, plastic trunking and some nails, bonding plaster, decorators filler & gardz