Paul,
Normally a food hygiene certificate only applies to food preperation/handing & serving staff.
It would be worth contacting you insurance company to consider upgrading your public liability insurance to £10 million.
On commerial cleaning of ovens, it maybe worth your while to see whats involved with duct & filter cleaning methods on a commercial scale, as i believe they have to be cleaned every few months or so for fire prevention reasons.
To get more info on these things, i would contact the "Health & Safety Executive" or the health & safety officer of your local council and ask them how they should cleaned and what else is involved.
good luck, let us know how you got on.
regards
keith b