<<as discovered by several Fairoaks fliers when LHR got their improved intrusion detection system>>
If you think you have strayed into a corner of the zone the best advice is to contact Heathrow SVFR and tell them you are unsure of your position. 999 times out of 1000 they will give you every possible assistance and you'll hear no more. However, if you wander about inside the zone and cause them to delay Heathrow traffic then you may have trouble. Many of the controllers are flyers themselves, ranging from basic PPL to ATPL, so they know what it's like to be a rookie.
Good luck...