Thanks to everyone for their replies. Sounds like it's a combination of pipework and unsuitable taps. Disconnecting the flexis and testing the flow rate sounds like a plan, so I'll try that and take it from there.
The bath flow was fine before (1970s taps and 22mm pipe)
The basin hot flow is 3.5l/minute. The basin has also been replaced and a new mono tap installed, using 15mm pipe and flexipipe.
The minimum pressure required for the new taps isn't stated, it just claims to be suitable for low...
I've had work done on my bathroom, with a new mixer tap installed on the bath and the hot and cold water flow from the bath tap is pitiful. The hot is coming out at 2.5 litres/minute
I have a older central heating system, with a hot water cylinder in the loft and my bathroom's on the first...