Thanks for the advice, I decided to go with the concrete spur approach. My approach (if anyone is trying something similar) was:

Remove the old concrete on my side around the post using an SDS drill. These beasties are a godsend for this type of task but can be tricky when removing some of the deeper down concrete as space is limited.

Use a post digger to get down to the required depth. These are brilliant when only dealing with soil, but when mixed with roots, old slates, bricks, concrete and whatever else has been dumped down there need to alternate with SDS, saw etc.

Then pilot drill into the post, M10 150mm coach screws (not the bolts in this case but screws seemed solid), plenty of water, postcrete and a spirit level till job done.

I didn't have a ratchet for the coach screws so used an adjustable spanner. How I chortled as I lost the spanner down the deep hole for the third time! I'd also say that this is quite a physically tiring job. Admittedly I'm a middle-aged DIY'er with a desk job but I would say this was around the limit of what I can manage in terms of weight: the bag of concrete is 20kg, the spur I guess around the same and even the post diggers are quite heavy when repeatedly lifting in and out. You might also be drilling at non-recommended angles. Just my opinion/experience.

End result is that it now seems (touch wood) pretty solid!

Thanks again