Try Myson?
"Can I drill out & re-tap bleed valves on old radiators?
If your radiator is a Myson Premier two tapping radiator, manufactured before June 2003, the air vent is in line with the rounded top of the radiator. This vent may be re-tapped with a No.10 32UNF tap (4.1mm drill size). For all other radiators, please contact the Myson Customer Services Office who can supply a new vent assembly that simply screws into the radiator.
Try here for more info
http://www.myson.co.uk/useful_services/faq_radiators.asp
good luck
"Can I drill out & re-tap bleed valves on old radiators?
If your radiator is a Myson Premier two tapping radiator, manufactured before June 2003, the air vent is in line with the rounded top of the radiator. This vent may be re-tapped with a No.10 32UNF tap (4.1mm drill size). For all other radiators, please contact the Myson Customer Services Office who can supply a new vent assembly that simply screws into the radiator.
Try here for more info
http://www.myson.co.uk/useful_services/faq_radiators.asp
good luck