a few links from google (I searched on "holding mobile driving") which imply / state that is the 'holding' which is the issue, rather than the 'using'.
First one is actually an exact replica of your situation
It looks like the key element is that it is possible for a policeperson to
see you holding the phone - but they can't necessarily
see that you are using it.
Therefore, it looks like the holding is the key.
I have
no idea if the phone records would prove (or disprove) that you weren't actually reading a text message?
(Not implying that you were, but if the cops believe you looked like you were doing, can you prove you weren't?)
Caught holding but not using a mobile phone while driving: should I plead not guilty? — The Roadside Lawyer
RAC Know-How - Owning a car - Safety & Security - Driving and mobile phones - FAQs (1st FAQ)
Driving - Consumer Advice from The Carphone Warehouse
(although this one is a link to the 2003 law, not the 2007 update)
swarb.co.uk :: View topic - "Using mobile phone while driving"the def of using
(another one very similar to yours)
There were others (200,000 hits), but that should be enough to give you a flavour...