Just did a Kellys Directory check for them:
Tel: (020) 8953 1995
Fax: (020) 8953 3029
Elstree Aerodrome
Elstree
Borehamwood
Herts
WD6 3AW
Their
Web Link appears not to be working at the moment.
[Edited to say that I just did a Companies House search and there is an Interiair Ltd with a registered office in Bristol that was dissolved on 22 Oct 02 - that is the only variant of Interiair registered in UK].