Better pictures now of the temperature sensor pipe routing. Picture at right comes from Road and Track magazine, December 1955 issue, showing original routing along side of cylinder head, and a few strain relief loops before attaching to the heater shelf.
The following pictures are from a gauge repair and installation in March 2018. The pipe runs along the head gasket line, through an easily dismountable special clip under the heater valve. This clip was introduced at (c)49148, but is recommended to be retrofit to earlier cars. The pipe continues then to a P-clip on the heater shelf (where I show an incorrect second factory special clip). It passes through the bulkhead near the control box (on LHD car). Be sure there is enough length and enough flex in the pipe to allow for engine shake between the heater valve clip and the heater shelf clip. You don't want engine shake to eventually crack the fluid pipe. I would prefer to see a 360 degree loop in the pipe where it approaches the clip on the heater shelf for additional flexibility. The pipe usually has excess length, much of which would be curled up and attached with a P-clip immediately behind the bulkhead panel.

The pipe passes through the bulkhead near the control box (on LHD car) with a special rubber grommet large OD small ID. This grommet should be installed on the pipe before the pipe is soldered to the gauge. Otherwise you have to make some radial cuts in the ID of the grommet to get it over the thermal bulb and flare nut. In this case the pipe was a bit too short and was taking a slightly longer route than normal (placing the temperature gauge on left side of dash), so it didn't have enough length to make the 360 loop. Original gauges had so much excess pipe that you could re-route from the sensor bulb to the inner fender with a 360 loop in between, then along the inner fender to the bulkhead, and still have a couple more loops behind the firewall.

Following are pictures of a very original MGA 1600. This one shows two full 360 degree loops in the pipe right at the front edge of the heater shelf, just before being attached to the heater shelf with the P-clip, then the path goes under the heater air inlet duct to the bulkhead near the control box, with one more P-clip along the way. Under the dash it has another P-clip and another 360 loop before proceeding to the dash and back of the gauge.

You can see the factory was taking great care to protect the fragile pipe to avoid cracking the pipe and loss of the working fluid. Do be careful handling this pipe during servicing of the vehicle. Illustration below is from the Service Parts List. The P-clip item 40 has a count of 3 with 3 each of the associated fasteners, with two clips on the heater shelf and one behind the bulkhead.