Just to update my findings. My two Screwfix sensors arrived and I can confirm that these are exactly the same as the heat pump sensors, the clip is different but can be easily swapped around (nice one @Dan_Grey 
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I tested my current sensors once I removed them and found a difference of 0.5k ohms which was equated to difference of 0.9 degrees. Once I got them all in one place I put them in a bowl of water to measure how close they were to each other:
Original sensor 1: 12.12 k ohms
Original sensor 2: 12.55 k ohms
New sensor 1: 11.91 k ohms
New sensor 2: 12.17 k ohms
I ended up re-using orig sensor 1 and new sensor 2 being only 0.05k ohms difference.
I then refit the sensors, made good the insulation and taped up around the sensors. Restarted the heat pump and now have a offline Delta T of 0.1 degrees between flow and return. Happy days
For reference, I struggled to get above 3.5 on the COP on the old sensors (return sensor was the higher reading of the two) but for the last 2 days I’ve been over 5 COP which makes more sense as my flow temps are generally really low and the house is always comfortable. The two images below are from today (so far) and 11th Feb were the outside temps were the same. Previous COP 3.4. New COP 5.48. I will continue to update COP readings to confirm, but a lot better than previous. It’s great to know that you now have two sensors calibrated closely to one another. Thanks for all the help on this.
