No response from Thomson, but I found a useful post from tghewett in this thread: "DHD 4000 - BBC Mux Problem on Mendip". Here is the interesting bit:
"I have my own DHD-4000 working now having had the same problems as you when Mendip switched over. The PSU can't cope with 8k mode and a full size hard disk at the same time. The only permanent solution I've found is to replace the disk with a laptop 2.5" one, using a large to small IDE adaptor to connect it. These smaller drives ignore the 12V supply and use less on the 5V, this frees the PSU to cope with 8k mode. Mine's been perfect since doing this."
I've had a couple of private messages with tg, and as a result I have copied his advice. I bought a Samsung Notebook Hard Disk 2.5 Inch Drive 160GB IDE from R U SMART TRADING LIMITED through Amazon for £42.50, and a 3.5" IDE - 2.5" IDE Laptop HDD Adapter Cable from CDL Micro again through Amazon for £3.99. All you have to do is remove the 3.5" drive from its plastic caddy, and disconnect the ribbon cable and the power cable. Next connect the adapter cable to the main board, and connect the power supply cable into the adaptor cable. I was unsure which way to connect the adaptor cable to the new 2.5 HDD, as this one is capable of being connected either way round. Obviously if you get it wrong you are likely to fry the HDD. Tg advised me to have the power connection at the opposite end from the four jumper pins on the HDD, or to put it another way, the power goes into the HDD at the far point from the front of the Thomson, if you have mounted the HDD with its label uppermost. If this is not clear send me a message and I'll email a photo. Once connected I fixed the new 2.5 HDD into the caddy using sticky tabs.
I have just tried it out and the reception is now greatly improved, and I would describe it as 95% of perfect. I still get the very occasional freeze or blockiness, and sound glitch, but not enough to be a problem. The reception had become totally unwatchable, but is now fine, if not perfect.
I'm going to try it out for a week, to see how good the recordings are on different channels, and then hopefully upgrade my other Tommy the same way. This way I can postpone buying new machines until the HD models become cheaper (and hopefully reliable).
The lack of EPG is still annoying, but I can live with it.
My heartfelt thanks to tghewett, who has been prompt and helpful with his advice.
Colin.