bt 1100 twin need to replace broken 2nd handset?

We bought a replacement handset to replace a broken 2nd handset. Phone in recharging base is identical and described as Gap compatible. We have reset the pin to default 0000 on the existing base phone and followed all the instructions to the letter several times but no matter what we do the new handset will not recognise the base therefore won't register. It displays PIN? then goes back to displaying BASE1 with the signal continually in search mode. Seller insists nothing wrong with phone we have just purchased. Could it be that the original second phone, now not working, should have somehow been de-registered from the base as this handbook states only 2 phones can be registered to at BT Studio 1100 at any time, and the original 2nd handset would have been pre-registered at the base?