nick smith wrote:
Thinking of having the boiler moved and one possible position would place the
outlet to the side of an openable window.

Its only a small 50000 BTU boiler and the original handbook says
300mm - is this still the current requirement ?

Depends on the boiler - fanned flue models have much more relaxed
spacing requirements than the onl unfanned balanced flues. Mfrs websites
often have online installation manuals with details of spacing.

Do you mean moving the existing boiler or replacing it? You _must_ have
the manufacturer's instructions in order to reinstall the boiler
elsewhere, and these will have the spacing instructions.