The 'motivation and interest' section is a series of questions on the information sent to you.
For example, you should know the airports at which NATS provide air trafic services and the airspace types. i.e. uncontrolled, controlled, and what type of airspace is above FL245.
I was unfortunate enough not to be able to get a hold of any of the suggested reading material but still got through easy enough with the information from the brochures (including the 'technical interview')
If and when you get through the selection tests, re-post here and I'm sure you will get loads of advice for the tech interview.
Luck
GW