I have soaked the nib section in clean water overnight and and jiggled
it about. After refilling with Quink black it appears to write OK
again. Writing in a soft stroking manner now puts ink on the page
whereas it did not before, and this is on cheap A4 copier paper.

I have learnt some lessons here.

Always ask people to give you the receipt with xmas presents
Don't assume all inks to be the same except in colour (from comments of
contributors and not from what Parker has said).

The topic has also raised a question: -

--IF-- this pen is covered by a lifetime guarantee, why does the
manufacturer need to see the warranty paperwork, surely it is covered
no matter how old it is? ;-)

Thanks again for the insights that you have all provided.

Take Care

Andy