Replaced bottom immersion heater but stat tripping

Hi
About six months ago I had to replace the grundfos pump which was completely knackered (with the correct pump) also replaced both PHE and DW sensors only to find they were wired incorrectly (which I corrected) found it had damaged PCB so replaced as well and all was well for six months until the bottom immersion  element packed up (tested with ohms reading) so replaced this yesterday and everthing was good but overnight bottom thermostat tripped (thermostat new came with immersion element) have checked connections everthing ok.

Top thermostat never trips out so am just wondering could it be someting to do with my horstmann timer or is there something i have missed!

This saga is starting to get a bit fustrating! Any help would be appreciated

Swop the stats round for starters.

Mike
MGC/H2O
Norfolk