I bought the car in Jan 2024 and this fault has always been apparent under 'fairly' heavy braking i.e on a motorway/duel carriageway slip, this is when the issue of the brake peddle pushing back is noticeable, but not always.

At first i thought it might be the brake servo faulty, then some research pointed to a sticky caliper. Sure enough the rear nearside caliper was faulty, so was replaced with new pads.

However, the peddle push back is still apparent, more research points to an ABS sensor mount rusting causing a misaligned sensor. However, I have never seen an ABS light flicker when the brake peddle pushes back. I would expect be an indication if the ABS was operated.

Has anyone got any thoughts on what i should be looking for to diagnose this issue please? 

Does the brake pedal feel spongy when pressed hard while the car is at rest? If so, it would be worth eliminating the possibility of air in the hydraulic system by bleeding it. Use fresh hydraulic fluid as old fluid can degrade as it absorbs moisture from the air.

The brake pedal is hard with the car at rest.

In that case I would agree a servo fault. Quite how to fix it I am not sure, so let's wait for other opinions.

I had a front brake caliper replaced and the brake fluid replaced. The fault has never reappeared.