From what I understand its best to protect outdoor electrics using an
RCD. Why is this an OK solution?

Basically, because there's next to no chance you'll get electrocuted by most
light fittings. I'd be more concerned if the light fittings are largely
metallic and not double insulated. I would generally say it was preferable
to RCD outdoor lighting, but is rarely essential. Note that TT earthed
installations always require an RCD, but there are methods to reduce
nuisance tripping.

No. I would not share outdoor circuits with RCDed indoor circuits.

Whats the reason behind this?

Because outdoor circuits are subject to moisture that can cause nuisance
tripping. When this occurs, it is best that it doesn't turn off all your
sockets, taking out your computer, TV and even fridge/freezer in some cases.

So is it better for me to extend my existing lighting circuit to
accomodate the downstairs lights or if not possible what can I do to
make a safe installation using the existing indoor RCD'ed circuits?

It would be OK on the lighting circuit, but best on its own circuit. It is
not possible to modify the indoor RCD circuit to remove the possibility of
the outdoor circuits providing nuisance trips.

I need to understand that if this is currently a spur from the existing
lighting circuit or the ring main how I can effectivly control two
installations from the one spur. As I understand it should be one spur
per fitting??

No, this would be fine. The lighting circuits need to be off a fused
connection unit, as lighting circuits must normally be fused at 10A or below
(some fittings must be 6A or below). Normally, you would use a connection
unit with a 5A fuse. This can then drive the dual gang switch, which should
either be DP (preferable) or have the neutrals looped at the switch. You can
then switch to suitable outdoor cable (i.e. SWA/XLPE/HiTuf) and run to the
light fittings. If you must run the lighting off an RCDed socket circuit
(knowing the risks), then definitely use DP switches, as some types of
faults are neutralearth. The DP switches will isolate the exterior lights,
meaning you can turn the house power back on, even without clearing the
fault (which might require sunny weather).

Your existing setup probably uses a switched FCU. If so, I'd replace this
with an unswitched unit and then feed into a dual gang DP switch. There's no
requirement for DP, but it is (very marginally) safer, and, more
importantly, provides an easy method to loop the neutrals without requiring
crimping or additional terminals floating around.

Christian.