Hi Paul,
Mercury trim pump for this model is made by oildyne,
Oildyne is a subsidiary of parker hannifin, many dealer over the world (hydraulic stuffs specialised)
https://www.parker.com/literature/Hy...%2012-2015.pdf
So if the modele you're looking for is this one in picture, then I've a link in France at a reaaly good price= 36.70€ incl VAT (you ned to add the ride)
the mercury cross reference is
Merc 883166A1
I've other link too for the older design one too (model with one screw)
or you can by the reservoir with the oildyne catalogue at any parker hannifin dealer in UK, no necessary to pass by mercury dealer.
And your PM box is full also.