BAA's site says ...

Hand baggage restrictions

Cabin baggage must be no larger than:

56cm (22in) tall
45cm (17.7in) wide
25cm (10in) deep

You should also check with your airline, as some operate further limits on bag size.
All items carried will be X-ray screened.
You no longer need to remove your laptop from your bag at Heathrow Airport.
Put all small items like keys, coins, mobile phones into your coat or hand baggage and place these into a tray.
Place your hand baggage into the tray.
If you have liquids in containers of 100ml or less, you can put these into a clear resealable bag. These items should be placed alongside other items in your tray.
Pushchairs and walking aids will be X-ray screened.
Wheelchairs will be thoroughly searched.
To save time, we advise you to place only valuables and items you need for the flight into your hand baggage.

So it looks as though 22" is OK for BAA. Hard to believe a jobsworth would know if there were any individual airline restrictions.