I sourced a replacement unit from a breakers yard, making sure its a good one simply by ensuring that there was no evidence of anti-freeze/white staining nearby.
Removing the unit is a right pig of a job, firstly getting the hoses off each end isn't made easy by the spring clips as they are invariably placed at a very awkward angle which meant that I needed to use several different types of pliers in order to compress the clips. Removing them in the breakers yard was easier as I used a saw to cut through one of the hoses, obviously that won't be an option on your own car!
To remove the heater unit I used several different spanners, especially stubby ones including stubby ratchets as the securing bolts run under the metal hydraulic pipes. You'll also need to push back/down the heat shield as the lip is higher then where the securing bolts are

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The heater unit itself is split in two (where its most probably leaking from) with a bolt from each half securing it to the mounting bracket so undoing the bolts holding the two halfs together won't allow you to split it without first removing the mounting bolts. On the original unit fitted to my car the heater unit to mounting bracket bolts were Torx bolts so with the lack of room there was no chance of getting a Torx bit in there, however the replacement unit used normal hexagon headed bolts so it might be possible to remove the unit from the bracket leaving the bracket in place, quite how you'd refit the bolts might be rather difficult though!!
Ultimately it can done but lots of patience is required, not to mention being able to balance yourself on top of the bonnet slam panel with your head pointing down the back of the engine

Rather than scour breakers yards for a replacement heater unit try
http://www.edwardsandson.co.uk/