Hi Simon
Its quite easy, you just need a switching jack socket like the one in this example
http://www.ebay.co.uk/itm/Connector-Jack-1-4-Chassis-Socket-Guitar-Amp-Mono-Switched-6-35mm-00620-/201177712534?pt=LH_DefaultDomain_3&hash=item2ed7204796If you look at the picture, there is the prong which normally connects to the tip of the jack plug, but there is also a tab which touches the prong when no jack is inserted (this is the switching bit). The circuit goes from the tip to the tab when no jack is in and the circuit is broken when a jack is inseterted.
You connect the ground to the, well ground ring as normal. (including the -ve or ground for the little internal speaker)
The speaker output from the amp is connected to the connection for the tip prong (as normal).
Connect your little internal speaker + to the tab connection that is touching the tip prong (until a jack is inserted). When a jack is inserted in the socket, the prong moves and the circuit between the prong and the little tab is broken.
Simples