Richard,
Did the battery on my 110 Td5 the other week. You need to loosen the battery connections, then turn the ignition on, then off and then remove the earth connection within 10 secs - think this stops the factory alarm from seeing the power cut as an attempted break in.
Reconnection is straight forward - just connect the earth and live as normal, no need to on/off the ignition.
The hardest bit is actually getting the old battery out of the battery box !
Jim