Oval conduit, back boxes and plaster board

Apologies for asking this again, I've spent the evening reading the forum and have almost got the answers - I think

I'm renovating a 1930s semi, I've the walls back to bare brick but need to move some light switches and add sockets. The new electrics will all sit under D&D plasterboard.

Current thinking is:
20mm oval conduit, fixed to wall, with clips & nails. Conduit will run from skirting to socket, and from ceiling to switches.
25mm metal knockout boxes with grommets to accept oval tubes into round holes.

Questions:
Do I need to chase the conduit in ~5mm to allow for the plasterboard?
Do I need to use larger (35mm?) knock-out boxes, any issues with making such deep holes in single skin brick? Especially as in some cases sockets will be near back-to-back with adjacent rooms?
Do I need the grommets?

Any comments welcome, be gentle!

I be going through the LABC prior to starting, but I wanted a head-start with the planning, I'll be reading the On-site guide too...

Why do you want to use oval conduit as opposed to metal or plastic capping ?

There is no advantage in using 35mm boxes for sockets if you dress the cables correctly in 25mm ones. Deeper boxes are an advantage for switch positions for the fitting of grid switches, dimmers etc.

You will need to be careful on internal walls.
Grommets should always be used

The house is an on going project, so spurs etc will probably be needed in the future, thought that would be easier with conduit.

To be honest you will struggle to get another 2.5mm (or 4mm) through an oval conduit and a grommeted hole once there are already two in it.

35mm boxes are only required if you want to have flat plate sockets - the type where they are completely flat with the wall.

Conduit won't make adding anything easier in the future, and if you are already considering adding items, why not put in suitable quantities of sockets now?

25mm boxes sound like they'll be ample for us.

Hmm, maybe PVC capping would be an easier/better way to go. Should 25mm be enough for single/twin 2.5mm² runs?
Any opinions on makes/models:
http://www.discount-electrical.co.uk/section.php/107199/1/pvc-capping
http://www.tlc-direct.co.uk/Products/MTCAP25.html

I assume you just nail it to the mortar joints using galvanized nails?

Thanks for all the help!

..

I can only see the conduit being off use when a re-wire will be needed in years to come, by then the DnD plasterboard will have most likely have lived it's live too.
I would just clip the cables and board out.