Ended up taking out the 'O' ring seal from the cap, putting the filter in and hey presto, it fits. It'll still seal, as there's a ridge inside the cap to retain the 'O' ring and this sits down onto the foam end of the filter.
Domestic water pump next and this had been leaking internally for quite some while. Every minute or so the pump would pulse to regenerate the pressure, a nuisance at night when you're trying to sleep.
Then there was the gas regulator, this was relatively easy to sort, turn off at the cylinder and remove the regulator and hose. Replace with new using new Jubilee clips on the hose. Tested for leaks using the approved gas detection fluid, all OK.
Oh! Hello. T time? Thought so.
Bathroom extractor fan tomorrow - stay tuned for another exciting adventure.