I've had a problem installing the hotend thermistor. I found the PTFE heatshrink when shrunk was still too large to allow the thermistor to slide into the heater block. I have had to carefully cut the PTFE off from around the body of the thermistor in order to slide it in.
Is it possible the PTFE heatshrink has changed spec in kits? Mine is RS kit number 472.
With the heatshrink on the diameter was 2.39mm, the hole diameter is 2mm. The thermistor is 1.78mm.

I had the same dimensions (themistor with heated heatshrink).
I used a sandpaper and grinded the heatshrink all around the thermistor.
required some time to get it to 2.15mm (fits now in the 2.05mm hole), but its very accurate.

My build is an early version and I initially found the fit too tight, so I did a final hot air blast (from my solder reflow hot air station) and made sure it was fully shrunk, this did the trick and I pulled the thermistor into the block using pliers. So check the heatshrink is fully shrunk.