Am currently doing a rewire and have a few questions please...

1. (the main question!). I'm proposing the following circuits - does
this plan sound sensible? BTW total floor area is 70m2; CU is a Volex
split-load item: http://tinyurl.com/7fe6m (Screwfix 82204). So:

Non-RCD protected:
a) upstairs lights, 6A MCB, 1.5mm T&E cable
b) downstairs lights, 6A MCB, 1.5mm T&E
c) linked smoke alarms, 6A MCB, 1.5mm T&E
d) downstairs ringmain feeding specifically the oven, fridge-freezer and
gas combi boiler, 32A MCD, 2.5mm T&E cable

RCD protected:
e) upstairs ringmain, 32A MCB, 2.5mm T&E
f) downstairs ringmain - all sockets other than those in circuit (d) 32A
MCB, 2.5mm T&E
g) electric shower, 40A MCB, 10mm T&E

I expect circuit (d) to cause most comment?! I don't see the need for a
dedicated cooker circuit because I'll be fitting a built-in single oven
with 13A plug (plus gas hob), which could be plugged into an ordinary
ringmain. OK?

2. I've got some spare 1.0mm red/black cable which I'd like to use up -
would there be any issue with me mixing-and-matching 1.0mm and 1.5mm
cable within one installation (I don't have enough 1.0mm do do the
whole house)?

3. CU is under the stairs, which means that the route for all cables
will be running under the stairs, in a straight line, at ~45deg, in the
space created behind plasterboard attached to battens on the bottom of
the stairs (clear?!.) Now, if the plasterboard were 'ceiling', that
arrangement would unequivocably be perfectly OK; however if it were
'wall', it would definitely be unacceptable because the cables would be
well within 50mm of the surface. What I'm unclear about is what does a
45-deg-mounted surface count as - ceiling or wall? I think it's
ceiling, but I'd like reassurance that some future inspector isn't going
to throw a wobbler at it! Not sure what to do if it's unacceptable as
it is.

Thanks a lot
David