Why is my pumped shower suddenly slow in starting?

[diybob]

Not been on here for ages, got a lot of help when I installed my bathroom some years ago. Everything went fine until I had to replace the shower mixer valve recently. The old one had started to get stiff to turn, so I fitted an identical unit to make the job easy - it's a cheap, minimalist "Harrison Mode" unit from a well known victorian supplier. All went well, until I came back from 2 weeks away, and I now find there is a delay when I turn the shower on - was a second or two, now up to 5 seconds before the pump starts and the water starts to flow. It's fed from a low-level H&C pump and has worked fine for over 10 years. Any ideas?