Potterton Performa 24 - Intermittent leak?

We have a potterton performa 24 which has been installed for about 4 years (very little use until 2 years ago). It has a slight intermittent leak (a greenish deposit) which comes from the joint between the 2 halves of the diverter valve assembly. Is there an 'O' ring or some other type of seal athis joint and is there a part number to order thhis item?
It should be quite easy to remove the DHW pressure diff. valve at this connection. The boiler is wall mounted, higher than the rads; if I turn off the cold waater inlet can I easily drain this into a bowl at the connection directly below the valve?
Any advice appreciated.

Thanks

John.