What's that big lump if it's not a reservoir?
I got one from BnQ - not sure of the make but it was a general DIY job - about 50-60 quid and worked fine. Fine, but incredibly noisy and I couldn't handle it and had to take it back.
I'm lucky enough to have a dentist air compressor - but I've used one without a reservoir and it didn't produce a good smooth result at all.