Thanks for any advice,

Good luck. It helps if the upstairs rads have working and well adjusted
TRVs. When these close down, more water is forced through the single pipe
system.

However, you may never be able to get the system working particularly
satisfactorily with the current pipework layout. The overall upstairs
balance may help. If the pipework is already in a suitable configuration for
such a valve, you may find subzoning upstairs and downstairs helps,
particularly if it is possible to have a pump for each zone.

Christian.