I think you need to give a bit more back ground info.

Why do you need to connect the radiator in this position?
What is the radiator intended to do?


I have a rear ground floor extension, that does not have any radiators
in it. It worked out that the existing rads in the open plan kitchen
and dinning room were adequate. But it does get very cold at the
furthest places away from the rads.

A new gas boiler is located in the extension, the pump is in the
ceiling of the kitchen. Space does make it easy to move the pump
closer to the boiler.

There is no way (i can think of) that such an arrangement would lead to a
correct installation.

The return pipe would be easy to T into, but its getting the flow pipe
that is the problem.

I'm thinking along the lines of 10mm microbore.