Fernox do a filler kit, Wolseley centre catalogue number 603120. at
£44 it could sound expensive but if you are using cleanser and inhib
every 3 years it cuts down on the hassle.

Near the top of the system, I have a vertical stub of pipe, followed by a
isolation valve and then a tupperware beaker with a tank connector on it.
You just empty the system enough, open the valve and pour in the chemicals,
not forgetting to close the valve before activating the filling loop!

I make that around 79p for the valve, 35p for the beaker, 85p for a tank
connector, £1 for a reducing T and a few offcuts of copper. That's 3 quid,
far less than you will save each time you fill by buying standard chemicals
instead of expensive "sealed system" concentrates.

Christian.