Inductive Load Switching and Interference

[hughgoodbody]

I am using a PIC 18F452 to control a proportional solenoid valve via an IRF530 MOSFET and PWM. This all works fine, however I am also using a Thermocouple to measure a temperature, this is processed via a thermocouple amplifier and then passed to the PIC. The problem I am having is that as soon as the valve is connected the temperature readings go mental and fluctuate all over the place, if the valve is disconnected, everything is fine and the temperatures are as expected.

I am assuming this is to transient voltages or suchlike due to the inductive load. I then powered the amplifier from the 5v supply (as this is not connected to the solenoid) and this made it a little better but still not good enough, again I assume this is because of the common ground. I only want to use one battery so probably cannot use optoisolators.

Can anyone suggest a suitable way of trying to solve this? A bit long winded I know so apologies!!! I have included a schematic of the relevant parts.

Many thanks and Merry Christmas!