The following page is directly copied from 'Riley's in Australia' http://www.phil.soden.com/Help.html, I have reproduced the content here because it matches the instructions from my local garage. I hope you find it useful, I certainly did, but unlike me don't forget to reconnect the dynamo when your finished - wondered why it wasn't charging!
A BIT OF NEGATIVE THINKING: CONVERSION TO NEGATIVE EARTH
The vast majority of cars produced in the last 20 years have negative earth systems and this is reflected in the limited choice of currently available electrical accessories, e.g. radios, cassette players, electronic ignition systems, which will accept the positive earth system used on RM's, and those that are available are often very expensive. This difficulty can easily be overcome by converting the car to negative earth, the procedure is very simple and should take less than an hour.
I have already carried out this conversion on two cars without any difficulty or special equipment. The longest part of the job is changing over the connectors to the battery if they have been soldered onto the heavy current wires. The battery itself should be turned around so that the negative terminal (now the earth) is -to the nearside. It is not unknown for this terminal to touch the bonnet top and if it is the live terminal the bonnet will be damaged. If it has not been done already it is a good idea to re-position the earth lead as that it goes to the starter motor fixing bolts instead of to the side of the battery box;
this often gives a considerable improvement in cold starting. There is no need to make any change to the starter motor, this will continue to turn in the right direction as it is series wound. Before the battery is re-connected the connections to the ammeter should be reversed at the back of the ammeter. No other changes to the instruments are needed, the fuel gauge and the clock will work quite happily with their polarity reversed (assuming that they worked before!). However, the opportunity should be taken to check the condition of the wire which feeds the clock from the ammeter as this is very fragile and is not fused. The next job is to reverse the polarity of the dynamo and for this the battery must be re-connected.
The polarity of the dynamo is controlled by the residual magnetism in the field pole pieces and this needs to be reversed by passing current through the field in the new direction. This can be done by holding open the regulator points with a piece of card and connecting a jumper wire from the battery live terminal (now positive) to the F terminal ( the small lower connection) on the dynamo or the regulator, whichever is most accessible.
The idea of holding the regulator points open is to prevent excessive current flowing through the regulator and dynamo armature. The jumper wire should give a small spark when it is connected. It is only necessary to let the current flow through the jumper wire for a second or two, after which the polarity should have been reversed. The piece of card may now be removed from between the regulator points. While this operation is being carried out, the points should not be closed by accident; if they are a very heavy current will flow and damage will result.
The final task is to interchange the low voltage connections to the coil, i.e. CB and SW, this will preserve the polarity at the plugs. The engine can now be started and all should be well. If the ammeter should show a discharge which increases rapidly as the revs. rise then the polarity reversal of the dynamo has not been carried out properly and should be repeated.
No other changes to the system are necessary unless later additions have been made, e.g. radios, electric washers, heater. Items such as wipers, horns, lights and electric petrol pumps will all work properly with the amps flowing the other way.