So I decided to replace the cells myself, my tools were a soldering iron, a T9 screwdriver, small sidecutters, 2 x 3" of insulated red wire and 1 x 3" of black insulated wire.
The original cells in my battery pack are 3.6v 4400mAh x 4. Replacement Li-ion cells came from a well known auction site were 3.6v 6600mAh x 4, so a +50% capacity. I bought 'tagged' cells which make replacement easier.
The four cells are configured in a series-parallel format to provide the required 7.2v @ 6600mAh capacity.
I noted the polarity of the original cells and then I de-soldered the original batteries from the charge protector which remains in the battery pod. I then arranged my new cells in the series-parallel format soldering the tags and adding the 2 x 3" of red and 1 x 3" of black wire. One red (positive) and one black (negative) wire are connected to the 7.2v output and the other red wire is connected to the 3.6v output - this wire is connected to the positve voltage supply for the charge protector.
Half an hours work and at a cost of £23.99 ............. a saving of over £100 !!
My next project is to replace the battery in my WM10 unit, this battery has its own charge protector built-in so will be much easier to do. The battery for this cost me just £6.78. Now I'm not sure if this is a repairable item, the 3030 battery pack is not, so its probable the WM10 is the same, however replacement of this is £180 - a lot of money if its just the battery!
This is how I repaired my battery pack, it is not a 'how to do it', so I will not accept any responsibility if someone does theirs and it all goes wrong.
Do not attempt this unless you are competent to do so.