Well said everybody.
No1 blade on RB211 and Trents is marked with an asterisk on the disc next to the slot the blade slides into, then add a piece of masking tape to each blade at the outer end and mark the numbers as you rotate the fan around. Don't mark the blade direct as won't come off!
In the event of a full blade set change then the replacement set should come with a patternation chart, which allocates each replacement blade to a disc slot based on the weight characteristics of each blade - this is created using a special programme in the shop and should ensure that only a trim balance will be needed to balance the fan to AMM limits.
Also when changing blades, don't forget to immobilise the fan!
RBT