Hi Geoff
Testbook is the diagnostic software system for the 75. It will run through a full series of tests on your car covering all engine, electronics and other systems and will produce a report. So it should tell you about any engine misbehaviours and locate the source of nay problem.
The trick with Testbook reports is to think about them carefully.
i.e as an example only - it may say your fuel pump is knackered - the garage then say need new fuel pump £300 please. We say clip has fallen off put new clip on, £5 all sorted.
If it helps and you get a costly diagnosis then do post up for a second opinion before splashing the cash. It could save you a few ££
hth