OK, turned out to be IC1.  Once changed I noted that both LED1&2 start to illuminate as volume and break-up occur, I'm thinking this is what must be clipping the signal to get the distortion - I've seen it done with normal diodes in a fuzz box circuit before.

Anyhow, job done. Thanks for the input folks.

I may have discovered the answer.... http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://www.circuitsonline.net/forum/view/31529&prev=/search%3Fq%3Dmarshall%2B5203%2BLED2%26biw%3D1118%26bih%3D584

sounds like a very similar problem, he doesn't say which op-amp he changed but at 60p a pop I'll do um both.. will post my results in a few days.....

It looks to me as tho the output of the second op-amp must have some sort of +ve bias which increases with the master volume (hence LED2 illuminating and not LED1).  But I can't understand how this is happening as the MV is on the output of op-amp3, it seems strange that it is effecting the previous stage!!  Oh, I wish I had a scope!

Thanks for the replies folks.

Yes, primofacprima it is a 5203, my bad...

Roly, zeners checked and they both have 15 volts across them!  I am also sure the pot is ok, the resistance increases smoothly when the pot is turned, no jumps or sticky points.... 

The Treble control has no effect on volume.

However, I have spotted that as the Master Vol reaches half way and the volume starts decreasing that LED2 begins to illuminate and gets brighter as the MV is increased (and the volume reduces) - I'm thinking this could be a clue?

the circuit board looks exactly the same as the one in this thread: http://www.ssguitar.com/index.php?topic=2851.msg21069#msg21069

Hi I have a Marshall Master Reverb 30.  It's a great little amp.  It has recently developed a strange fault - when turning the amp up using the master volume the amp gets louder but as the pot goes past half way the volume decreases?!?

I have removed the mater vol pot ad checked it with my multi meter and it looks fine.

any help, advice etc would be much appreciated