Hot water to new shower takes variable time to come through

[crlnhogg]

We have recently had a new shower installed. It is situated about one metre from the immersion tank and the cold water tank is in the loft two floors above.
The shower has a thermostatic mixer but is not a power shower. It worked fine for a few weeks after installation but then occasionally the hot water took between a minute to 5 minutes to come through. Once it was throught the shower worked fine. Now it is taking from 1 minute to 15 minutes to come through, though just occasionally it starts first time! Seems to us that the longer the gap between usage, the greater the likelihood that the hot water will not come through straight away. Cold water comes instantly.
We are complete plumbing novices and the installer has drawn a blank so any ideas would be much appreciated.

[swidders]

Suspect the mixer has developed a fault. Have you still the receipt since should be under guarantee.

[TheDoctor5]

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