The circuit cables will leave the consumer unit via trunking on the wall. Is it best practice for the cables to exit the consumer unit by the gap in the back plate or through the side walls? If the back then is there any preference to mounting the unit on a pvc or wooden frame type box to allow easy transition to the trunking ? Thanks.

Cables should enter a switchboard enclosure from the top, bottom or rear of the unit, You could mount the enclosure on a panel 100mm off the wall and have all the cables enter from the rear.

I would arrange the trunking to go along the top of the CU (usually where they have the biggest space for wiring) then slot out the side of the trunking to match up with the knock outs in the CU.

You may need to make a spacer from 10mm ply to fill the gap between the trunking and the CU so you can get the lids on both, paint the spacer to match the CU for a professional finish.

You will likely have difficulty getting all your cables through the available knock outs in the sides of a CU.

Is the other side of the wall available and invisible? Why not run the trunking up the back side and enter the CU from the rear?

Trunking and unit are under the stairs and out of sight and the otherside of the wall is the kitchen. I could run the trunking across the top of the CU and then block the end of the trunking to keep the geckos out, but I thought mounting on a panel and using the rear knockouts would make it easier to manage the cables to the MCBs within the CU.

Any preference for a mounting panel - wood or pvc ? I gave sealer and spray foam to stop the geckos slipping through any gaps...!

If you do mount it on a panel make sure you use something either fireproof or treated with a fire retardant, something like this http://www.panelsource.net/products/PurekorFireRetardantPlywoodFSCCertified.html and no, I've not seen it here

I would still run the trunking across the top, you get a bit of extra space for managing the cables inside the trunking if you use all the knock outs in the top of the CU.

Get the proper blanking plates for the open ends to keep the wildlife out.