And lo, it came to pass. I had a call back from OTE yesterday, who said that they gathered that I wanted to reduce my plan back to the original 4 Mbts. I explained carefully, as I had done in my previous call, and my email, that I WAS getting around 5 Mbts prior to their engineers turning the speed down to 3.5, and that I wished to be returned to the best speed they could offer, and that they should credit me with money for the time I had on the lower speed, whilst paying for 24 Mbts.
From this morning, I have a steady 6.7 Mbts. Simples!
Thanks for the info Martin. You are aware of my trade I hope? My incoming phone line was brand new from a new pole erected when we bought the house. Where it enters my office, it is via a twisted insulated, and quality screened telehone cable. The copper at this point is soldered, inside an IP box and coated in silicone greese. At the other end of this cable run (Around 5 metres) is the outlet. Again the best quality available. There never has been line noise, and the signal is always clear.
The whole fiasco was down to OTE turning the speed down, and at last, back up.