6l is pretty small Jim, I use a 24l reservoir on mine and that isn't to bad as the pump doesn't have to cut in to often (annoying the neighbours to...) Machine mart was where I got mine
http://www.machinemart.co.uk/product.asp?p=000111845 and I know it is a little more than what you want to pay, but unless you are willing top buy a 'silent' one, then you would be better off buying one that doesn't need the pump running to much (apart from the noise, they also have a max 'on time' and min 'rest tiem').