Time Machine problem after restoring to a new HD (after my original HD failed)

I am running Lion 10.7.5 on a MacBook Pro Late '11

My HD was getting SMART errors, so I did a complete back up using Time Machine using a WD My Passport 1TB external, and then sent my MBP to Apple for HD replacement (under my Applecare plan).

I collected it yesterday and restored my system using Time Machine (using the data from my external HD) and everything was restored perfectly, which was great,

However, I'm now encountering a problem after plugging my external HD back into my MBP on the new system. Rather than Time Machine doing a swift 'top up' back up, it appears that TM is attempting to do a full back up. (I have 300GB and it took nearly 24hours to backup on the faulty drive) Is this normal after installing a new HD? Bearing in mind the Time Machine data is still on my external drive?

It's also appearing to be taking FOREVER to do this entire back up again. It is stuck on 'preparing back up' and when I do get some movement, it's doing a couple of KB's at a time and with no genuine back up progress.

Fundamentally, I just want to just top up my Time Machine back up without having do a complete back up, again. (which doesn't actually seem possible, viable at the min)

Any help / support would be much appreciated 🙂