Is the plaster sealer sold in bottles and tins, eg B&Q Interior
Plaster Sealer, just an expensive way of buying water with a bit of
PVA mixed in? Couldn't I just use say a 1:3 PVA:water mix? Would one
coat be enough on new plaster before painting?

Use Dulux Trade White Supermatt, diluted 25% water, 75% paint. Follow by 2
coats of coloured Dulux Trade Supermatt. This paint is specifically
formulated to enable new plaster to breathe. Personally, I wouldn't use
anything PVA, or even use a standard vinyl emulsion.

http://www.duluxtrade.co.uk/webapp/w...sheets/410.pdf
http://makeashorterlink.com/?T1CC23D89

Christian.