shower dribbling when running a bath

[RedBugMug]

grohe quater turn bath filler/shower unit.
when running hot water it's fine but when adding cold the showerhead dribbles.

any trick to fix it?

I have a toolbox and am not afraid to use it :o

@pigletjohn?

the knob that you use to divert water between the outlets, it might perhaps be a bit loose, but more likely the valve is worn. You could ask the makers if there is a replaceable part. When you find out the price, look up prices of spare parts on Bristan, if you want to be annoyed.

If the cold pressure is higher than the hot, that might contribute. Have you got a hot-water cylinder? what colour?

thanks, combi boiler, no water tank. hot water pressure is pretty good, cold only slightly more. this issue is quite new (bathroom installed in '13)