If you decide to use a plans drawer, it is a good idea to ensure that he has appropriate insurance cover for his work. Chartered architects are required to have this as standard by the RIBA.
Your quote seems quite reasonable for the project, it is worth making sure that your architect is ARB registered, and confirming what the scope of work is - will your architect be prepared to instruct a builder initially or is it just a basic drawing service?
If an architect tells you that he is not registered, it is likely that he is not be an architect at all, but simply a plans drawer. 'Architect' is a legally protected term, and all architects must be registered with the ARB.
1000 pounds per square metre is a good ballpark figure for an economic 2.5x5m house extension, however, it pays to get lots of quotes from trusted builders in the current climate: some quotes may be quite low as they are keen to take on more work...