Manchester Helicopter Charter Company used G-BPAI primarily for pleasure flying from their base at Manchester Barton around that time.
Apart from Barton, they would operate pleasure flights from pub gardens in the summer.
Are you sure the snaking river is not the adjacent Manchester Ship Canal which runs alongside Barton aerodrome, and your 'pub' was in fact the clubhouse restaurant ? There are also a few hotels further along the canal which host helicopter visitors.
A colour scheme might also assist. BPAI is predominantly red. The reason I mention this as one of the MHCC shareholders had another Bell 47 at the time in an ex-Army overall paint finish, should that jog your memory.
I like your thinking, but I'm 99% sure it wasn't Barton. At the time I was a complete aviation nerd and wouldn't have been able to take my eyes off any other aircraft onsite. I'd definitely have remembered a control tower and airfield. Also, Barton is right next to a built up area - as I recall we were out in the sticks with lots of open countryside visible once we were in the air. It was only a very short flight, maybe 10 minutes, if that. Can't have gone far. I distinctly remember a large beer garden with your typical wooden benches.
Unfortunately I have no memory of the colour scheme, except that I think the tail boom was either a natural metal colour or painted silver/white. G-BPAI looks to fit this description, and if they were doing pub pleasure flights around that time, I guess could be a good candidate still.