The exact pump location does not really change the requirements for the
tank connection. The pump instructions normally give a range of
connection options that start with a dedicated essex flange as the most
deireable, to using the existing top connection as the least. Having
saif that in the right circumstances using the existing top connection
can work fine.

Thanks John. I've re-read the instructions again. The Essex flange
with the anti-gravity loop is compulsory if the pump is fitted above
the outlet from the hot water cylinder. If the pump is fitted below
the outlet then I can use any other approved top entry flange. These
include the Salamander (S) Flange, York Flange and Warix Flange. I
thought I had also read Surrey flange but I may be mistaking this with
the S flange. I figured it would be easier to use one of the approved
top-connection flanges rather than trying to drill a hole in the side
of the cylinder and the associated difficulties of fitting it.