Adapting a Computer PSU

[pamphonica]

Previously when I tried my transceiver with an "Avair AV-2025D", I had purchased specifically for this use, the DC negative supply to the transceiver was connected to the mains supply earth. This had the unfortunate result in the RF signal leaking from the coax into the mains earth and going all around the house. This was tested by using a non earthed 13 amp plug and lead to supply the PSU, which cured the problem of RF leaking into the mains earth.

Going back to the original problem with RF on mains earth, I suspect that you are feeding your dipole directly with coax. That immediately unbalances the dipole and leads to lots of RF coming back down the coax outer, and hence to earth.
Jeffrey has already suggested a common-mode choke, which is a good idea for stopping rf on the sheath.
The other thing is to put a balun at the feedpoint of the dipole. That will ensure a balanced feed to the antenna, but a nicely screened and matched feed up to that point. Worth a try, surely. Especially as the power you are using goes up.
There are lots of plans for baluns, using either toroids (use the T-200 type for 100W+ rating), or a bundle of ferrite rods bound together (can be very effective). You need a 1:1 balun.

The new psu does look very smart though!

-Jeremy