The white plastic bit looks damaged. Make sure none of the broken bits are inside the heater.
If it's blown off once then it probably will fail again. Think of a backup for tomorrow should it fail, i.e can you bypass the heater so you can finish your work? Can you just go home and have a couple of days off while the new parts arrive? Is there a canal boat repair place nearby that might do webasto spares?
See if you can find an exploded view of the heater and see if you can work out why it failed - ie. has a pressure relief valve got stuck with the frost etc.
Look at alternatives for the problem - is your controller dead ending correctly and not over pressurising the heater, any possibility of ice causing a blockage.
Don't just assume you have fixed it!
Last year, no one had stock of those mixer valves; not even the go-to supplier in Turkey.
The solution was a standard mixer valve, which is more readily available from plumbing outlets. These are fine for canal boats as they have a much lower hot water temperature. 46 degrees is the hottest you can get, where we need a much higher temperature output at around 60 degrees C. These are usually mounted on the hot water cylinder in a canal boat and once adjusted, isn't usually changed again.
https://www.butlertechnik.com/eberspacher-or-webasto-thermostatic-mixer-valve-tmv-kit-for-3-4-thread-plate-heat-exchangers-p3453The other issue is the orientation of the adjuster k nob. On our systems, the heat exchanger is inside the heater boxing with the control k nob outside so we can easily adjust the output temperature. If we use one of the Pegler mixer valves, we need to remove the front panel of the heater to make that adjustment. Is it the end of the world? No, because I can't remember when I last adjusted the output temperature. But still, its not ideal.
One of the other issues is pipe orientation inside the heater box. Grippatank do pack their heaters very tightly, so making piping modifications could well mean modifications to the heater hoses.
