4.7l incl. filter
that should be fine for flushing purposes!
iirc the JTDs should be ran on 5w40 Fully Synthetic
IF you dont mind loosing another £8 then
THIS stuff would be more suitable. i wouldnt worry too much about quality if i were you as it wont be staying in for any significant length of time.
if you can physically look at the bottle there should be an 'API' or 'ACEA' classification rating (to all intents and purposes, a quality rating). you should be looking for an API of CF or an ACEA of B3. there may also be an 'S#' rating but that is a petrol engine classification.
so to recap
4.7 liters of 5W40 Fully Synthetic Diesel Engine Oil (CF or above)
sorry for waffling (although the stuff in your link should be fine for flushing if its a diesel engine oil)