As you may know the R/Hand cylinder head runs at a higher temperature than the L/Hand one.
Before I started I checked the temperatures of the 2 heads when at running temperature.
The L/H was 43° & the R/H was 70°. After the mod they are both at 43°.

You will need some copper pipe fittings. An equal T piece, a 90° bend and some straight pipe.
I used end the fill type as they’re neater. I used new hoses and Jubilee clips.
My last car had 15mm i/d but the ones I bought were 12mm. Check before buying copper pipe.
You will have to modify your right hand Transfer Housing 218436 by drilling and tapping the blank hole facing the centre.
IIRC it’s 1/2“ BSP but may be wrong. You can buy one already tapped but they’re about £90!
You will need another Adaptor Hose 138530 to fit this hole.
You will also need a new Transfer Housing Gasket 151310.

See photo. I fitted a valve to my first one to stop any air locks but it wasn’t necessary so my present one doesn’t have one. Just make sure that the hoses are full before fitting the last one.
When I did the plumbing I cut back the joints between the T and the bend to make the space between them shorter.

Taking the water transfer housing off isn’t easy. I took off the hoses first.
There is a recessed bolt in the middle of the housing which isn’t easy to get at.
I cut a socket short to get at it.
Cut a piece out of the pipe going from the L/Head to the Heater to make room for the mod.

Danny’s mod:

In addition, if you have a MK1 Stag the right side Transfer Housing will have a large headed threaded Hex plug fitted. This was removed using a 11/8" socket, it is a tight fit to get access but manageable. Once out I searched around my spares and found a TR7 water pump cover, the outlet Adaptor Hose (Part No UKC1802) from this is is the same thread as the Hex plug.
Here is a photo of the parts I used. The hose is also from a TR7, I used the bend section.
I cut the Stag heater hose midway from Left Head to the Heater connection to fit the T piece which is also made from 15mm copper pipe.

If you have a MK1 this is a fairly easy and cheap mod.