There's nowt in this country to help thee marrer (Cumbrian too !!)
Nearest best practice is NFPA 96 2004 (now upgraded for 2008) in the good old US of A
The below link is from a cleaning company in the U.S. It outlines the various NFPA requirements:
http://www.chdca.com/nfpa96.htmThe only advice I can give is check with your insurers as to their requirements. I know for a fact that some commercial insurers will declare policies null and void if the build up of grease residue is too high for their liking. What that level of build up is will be for them to decide.
The HSE is pretty vague too:
http://www.hse.gov.uk/pubns/caterdex.htmhttp://www.hse.gov.uk/pubns/cais10.pdf......be easy to clean, avoiding the build up of fat residues.............etc. etc.
http://www.hse.gov.uk/pubns/cais12.pdfDoesn't even mention hoods, plenums and ducts in its' recommended schedule.
However, your hood extraction system will fall under HSE PUWER regs and should be maintained accordingly. Maybe the hood manufacturer may be able to throw some light on the issue ??
When I design a kitchen canopy fire protection system (and I do lots). I always specify that the canopy should comply with The LPCBs' LPS 1263:Part 1 (Fire Performance Requirements for Kitchen Extract Systems).
But this is more of a design criteria than a cleaning / hygiene one.
On a personal note, you'd have thought after the two big burn outs we've had in this country that have been directly linked to hood extraction, someone would have come up with some de-greasing guidelines or ACoP's by now.
So I'm afraid it's one big grey (or manky, greasy brown) area of nothingness in the UK. :/
Insurers would be your best bet.