The curent status is as follows.........
"By undertaking training in Unvented Hot Water Storage Systems, you no longer need to contact your local Building Control Officer to approve your work. You will be recognised as being competent to design, install, service and maintain Unvented Systems...
I would recommend likewise with the 25mm MDPE but i would retain my current stop-tap.
I would run the MDPE up to short of the stop-tap where i would use a 'Plasson' fitting to convert from plastic 25mm to copper 22mm. The remaining pipe-work not changed.
Good-luck
Part L' will be what your man was explaining.
He was trying to do you a favour, why not look into it yourself and realise when someone is trying to help you out.
http://www.idhee.org.uk/TACMAGuide.pdf
As you will see, it would be a good idea to accept his suggestion