yesterday the the waterboad came round and fitted water meter under the sink. Since then the boiler keeps coming on and going off. in the attic the expansion tank keeps hissing every now and then boiler baxi WM51/3RS

Joan, February 2020

Sounds like there is air trapped in your system caused by the meter change-over. Is there any way of contacting the water company to complain that their engineer has caused this fault and see if they can offer a way of rectifying this 'damage' they've caused? Just a thought...
If it is an airlock issue, I've found a link that may help you if you have the skills to carry out the necessary works required to vent the system yourself:

https://www.diydoctor.org.uk/forums/system-venting-t9456.html

Rupert, February 2020