Phil
I looked at the aerial view so I understand the 'shape' of what you mean.
We are probably getting close the limit of advice that I can provide you with, though I'm happy to answer further questions.
I'm going to provide you with a link to a specific piece of case law on AP - Pye v. Graham which was a landmark case heard in the HoL in 2002. As I'm sure you know every case it different and although circumstances of one determined case (especially at H o Lords) inform future cases down the line, the individual circumstances of the case have to be taken into account.
http://www.propertyla...sessionpyegraham.html
In Pye v. Graham, the claim by the squatter Graham was upheld, however the boundary was separated by hedges (yours isn't - the disputed boundary is a line across the grass).
In Pye v. Graham, the land had been completely contained at all material times (yours isn't - you have continued to gain access to the disputed land to trim the hedge - which you regard as in the midst of your plot) - can I ask - how tall is the fencing at the edge of the grass, between the back of the land behind the flats and the road on which your house is?
But these are legal points on which you need proper legal advice. As a fist step, you may be able to get free/modestly-priced but good advice from the property law website if you register there.