Hello Ron
I presume you mean that effectively you have a simple blanking plug as a "blowdown". Obviously this cannot be used as a means of blowing the boiler down when it is still in steam. It is desirable that this can be done as it will enable the debris and accumulated sediments to be blown out of the boiler under pressure. With a plug, if I understand your situation correctly, you will have to wait until the pressure has entirely disappeared before removing it. The boiler water will then dribble out, leaving a lot of sediment behind. Undoing a plug fitting with the boiler in steam is not to be recommended. You would need a good supply of replacement plugs and it would damage the threads in the boiler when it blew out!
Over time, sediments would accumulate in the bottom of the firebox and could lead to premature boiler failure due to overheating.
It should not be too difficult to fit a proper blowdown – there are lots of them described in ME and other magazines, and they're available from suppliers like Blackgates. You may need to provide a bigger hole in the frames to clear it, however.
The "Green Book" does not insist on blowdown valves to be fitted. However, a Boiler Inspector is quite within his rights to refuse to issue a certificate if in his opinion the boiler is inadequate in some way, or if it contravenes Club Rules. The Boiler Inspectors have your own interests at heart in this case – so don't despair, just fit a proper blowdown and everybody wins.
I think (in this instance) I may well have issued a certificate with the proviso that a proper blowdown valve be fitted within a fairly short time scale.