Vokera Linea 726 Low hot water flow with 2 taps open?

I have a Vokera Linea 726 combination boiler which has worked okay for around 3 years. Recently I have found the hot water pressure to be very weak when more than one tap is running in the house and find it takes around 15 minutes to run a bath.

I have measured the flow rate to be around 4 litres per minute but this drops to around 1 litre per minute from one tap when 2 taps are running.

Temperature remians warm at both taps.

The boiler pressure has not dropped or changed and remains around 1-1.5 bar.

I have also checked for leaks in my system and although i found 2 radiators were leaking very slightly, I have repaired them and the problem remains.

I tried cleaning the inlet filler pipe (silver braided connection) which was mildly dirty but not overly bad and also tried running the system without the coloured filter washer which helped a lot however i do not wish to run the system without this filter.

I have considered switching to a higher flow coloured washer filter however I am loath to do this as the system worked fine until recently.

Can anyone suggest where my problem may be?