Hi need some help the heater in my Marlin wont turn off on hot all the time, would be ok in winter but nit in hot weather, had a look cant see anything wrong, my question can join both water pipes together and do away with the heater? the engine is 1600 pinto 1991, has a water heated inlet manifold, many thanks.

Yes, joining the ends together should work, but you can get a cable operated changeover valve to do the job:

https://tinyurl.com/London-taxi-heater-valve

I can't tell if the following works in the same way, but it does look like a diverter valve:

https://tinyurl.com/Ford-KA-heater-valve