An oil fired boiler with the bathroom rad connected to it, but the rad does not heat up.
Indirect system, the pipe for the expansion tank is taken off the top of the HWC feed and into the loft. In the loft is a T and the feed for the rad comes off the T. The return for the rad goes through the bathroom and into the utility room and then to the AGA.

I have removed the rad and vented feed and return of any air, there wasn't any.
On reconnecting the rad and then venting it I opened the feed valve and removed the bleed screw. The pressure from the feed was a dribble from the bleed screw hole. On opening the return and closing the feed I got a horizontal jet from the bleed screw hole, suggesting greater pressure in the return.
An AGA plumber (via a relative) reckoned it was blockage in the system which caused the lower pressure in the feed and advised using compressed air to clear it.

I'd appreciate advice on which type of compressed air device to use.