Under the Clean Air Act 1993, Watford has been declared a Smoke Control Area
Smoke Control Areas are locations where you can’t emit smoke from a chimney unless you’re burning an authorised fuel or using ‘exempt appliances’, for example burners or stoves. You can be fined up to £1,000 if you break the rules.
Outdoor ovens, burners and barbecues
You can use outdoor barbecues, chimineas, fireplaces or pizza ovens within a Smoke Control Area, however any of these appliances that release smoke through a chimney of a building (i.e. a summerhouse) can only burn authorised fuel or must be exempt
Visit https://www.gov.uk/smoke-control-area-rules for a list of fuels which are authorised for use in Smoke Control Areas and for list of appliances which are exempt.