inner case seal for a stelrad w2000 wallmounted gas boiler?

where can i obtain a seal for the inner case of a ideal stelrad w 2000 central heating boiler,as british gas homecare say it is `missing` after 4 years of their maintainance and have capped it off,even though my daughter in law has two children under 5 and is 5 months pregnant.the boiler works fine otherwise.they inspect the boiler even though the case cannot outer case cannot be removed fully because off a kitchen cupboard,which can be removed by request.