Tips for writers:
if your main character is an electronics expert, you should get someone who's actually an electronics expert to check all the things he says about electronics, otherwise unintentional hilarity will ensue;
if your main antagonist is a smart, unstoppable terrorist, you should avoid having her repeatedly do really stupid things that work only because she's incredibly lucky;
if you're going to set the denouement of your book on the city walls of York, you should consult a travel guide to ensure that you don't miss important details about how placenames there work.
(Specifically, comments on cryptography and radio transmitters working two ways by accident, absolutely everything Witch does after her return to the UK, and confusing Monkbar, a gate, with Goodramgate, a street. I suppose it's possible to climb Goodramgate, but only if you're very drunk.)
That said, I'm not convinced that this was really meant to be anything other than a trashy thriller...