it is an easy DIY job
DIY? why not?
turn off the valves at each end of the rad. One of them will usually be a lockshield, meaning it is supposed to be adjusted, and then a cover screwed over it to prevent it going out of adjustment. When you close this one, carefully count how many turns it takes so you can put it back in the exact same position when finished.
When you have tightly closed both valves, no more water will come out of the bleed screw hole. You might then find you can screw the pin back in. If it has fallen into the key, you might need to winkle it out with a pair of tweezers. Does the pin-hole sit in a square or octagonal socket? this is important.
As you have drained the pressure, and some of the water, out of your system, has the boiler stopped working?
please describe your boiler. In particular, has it got a pressure gauge on it? Or is it an older one with a feed and expansion tank in the loft?
BTW if you are paying someone to come to your house and make a repair, remember that you are paying him from the time he takes your call and gets into his van, including driving to and from you. You are also paying a tiny proportion of the cost of running his van, and of his business costs, training and equipment, office, even a tiny part of his holiday pay and pension.
