My Marshall JCM800 2203 amp head was switched on at the mains with both the standby and power switches set to the "on" position (by a non-guitar playing friend). There was a big blue flash from the head and now the amp is dead.
Can anyone give a rough idea as to what has potentially blown and whether this is repairable (by a professional, not into DIY fixes personally).
A fuse blowing can make a flash like that, but its more likely you'd notice it in a dark or semi-dark room. Have you checked the fuse/s?
If the amp had the correct fuse/s installed, there no reason that anything else would have been damaged. In fact, normally that situation (fully "off" to fully "on") wouldn't have even caused a fuse to blow.
You may have blown a power tube, which would probably then blow the fuse. If that's all that happened, than you'll need a new set of tubes and a new fuse, which is easy for a tech to do. If it's more than that, a good tech should be able to figure it out and fix it.
There is very little that can't be fixed on an amp by a good tech. This is something you don't want to just take to whatever music shop is local, find a certified marshall repair tech and go there. If they don't have a listing on the Marshall web site than email them and ask where you should take it.
As mentioned, you probably blew a tube, but take note...the amp should be able to handle being switched on to full power. It may take some life out of the tubes, but not blow them. The tube was probably weak already, or had a defect which caused it to short when powered on cold.