At Risk by Stella Rimington

From the real-life ‘M’ of the James Bond films.

For MIS Intelligence Officer Liz Carlyle the nagging complications of her private life are quickly forgotten at Monday's Counter-Terrorist meeting. An invisible may have entered mainland Britain.
An 'invisible' - a terrorist who is an ethnic native of the target country, who can cross its borders unchecked and move about unnoticed - is the ultimate nightmare.
For Liz this signals the start of an operation that will test her to the limit. Who or what is the target? Where and who is the invisible? With each passing hour the danger increases. But as she desperately sifts the incoming intelligence and analyses the reports from her agents she finally realises that it is her ability to get inside her enemy's head that is the only hope of averting disaster.