it has become obvious that Orange are unwilling to re-instate my original speed against my current contract.
I'm sorry to hear that, it seems vastly unfair. Most decent ISPs have upgraded their customers at no additional costs and dont require the customer to enter into a new contract to get the higher speeds
>> We all know it's technically possible to re-instate my original speed
Yes its a restriction at the ISP end (not the exchange equipment) which will now be cutting your speed back.
I think you may be correct, more and more ISPs tend to be moving towards the packages where you get everything all bundled in, not realising that some of us may not want that :/