As has been suggested S-23 should be available from most LHBS
I prefer W34/70 but it can be difficult to get.
s-23 is a little bit sulphurous though . . . . so remember its the yeast that farted!
if pitching cold ( <16C ) use at least 2 packets in 25L
Hop Tea - Take two tablespoons of extract from the kit dissolve it in a litre of boiling water, then add 20g of Hops (Saaz is a great choice). leave it for 20-30 minutes (while making the rest of the kit up) then add back to the kit. Gives it a good boost.
One thing I've noticed with the Coopers Lager kits is that I find them better made up with Sugar and not spray malt. Even the extra light throws the colour and balance of the kit off