This failure was due to the fact that, like many other modern mobile phones, the HTC needed a capacitve stylus. The local mobile phone shops weren't much help, but an internet search and poll of friends located the following 'pen' like stylus from Acase. The pencil grip fitted tightly onto the new stylus and I was soon back in business. Here's the updated hack pictures.
One place that a capacitive stylus for HTC mobile can be found is Amazon UK: http://www.amazon.co.uk/Acase-Capacitive-Stylus-Galaxy-Screens/dp/B003ULE6GU/ref=sr_1_1?s=electronics&ie=UTF8&qid=1312912947&sr=1-1
The pencil grips can be found here on Amazon UK: http://www.amazon.co.uk/WRITING-GRIPS-GRIP-PENCIL-PACK/dp/B002GZ796K/ref=sr_1_2?ie=UTF8&qid=1312916637&sr=8-2
I'd be most interested to hear about any better solutions to the stylus problem or different versions of grip.
Hope that this 'hack' is useful to someone.