I'M TRYING TO CALACULATE WHAT SIZE BUOYANCY BAGS TO purchase. I'm thinking of providing 180litres of buoyancy through 2 x 48"x14" bags. 

I've heard its usually 2.5xweight of boat - can anyone confirm? 

IT WOULD BE HELPFUL INFORMATION TO ME IF ANYONE KNOW ROUGHLY HOW MUCH BUOYANCY CAPACITY A BOWTANK (SEPTEMBER GIRL HIGH TANK) HOLDS IN LITRES? 
any help much appreciated

48x14 is much larger than standard which i think is 36x12. This has been standard for many years (Every merlin i've ever owned has had it) so i'd tend to stick with it.

I'd guess that the bow tank would be about 50-70 litres.

if you are thinking of adding a bag to the bow tank, i have seen a smaller size holt triangular bag in a september girl tank, but this was done to get round a leaky tank. trouble was it trapped the water in and made the rot worse. leave the inspection cover off when stored so it can breathe, or better still, it may be time to get out the epoxy and solve the leak!

It's difficult for me to work it all out in metric particularly since the writing on the bags in my boat happily mixes imperial and metric on the same labels but I have 2 x 35"x11" bags at 45Kg each under the quarters (36"x12" give an extra 5Kg each)supplemented by 2 x 24"x7" bags at 18Kg each on either side between the quarters and the centre thwart (very comfy helmsman's inboard seats!) and my built in high bow tank measures roughly 100cm long x 90cm wide x 30cm average height. I think that gives a volume of approx (and at least) 90 litres which (if the conversion is 1 for 1) would be 90Kg. This would equate to being able to stuff the 2 inflated quarter bags inside it (which I could do easily). So that gives me 216Kg. Not quite 2.5 times the nominal weight of the boat but if it is made of wood and plywood it should be inherently buoyant. 3112 floats quite high as long as we remember to close and insert the bow tank seals and bungs securely before setting sail!
Easy to say don't capsize so you won't have to try it all out but not so easy to achieve!