It is almost impossible to glue the original back especially as it usually warped. Reasons why can be found here:
http://forum.etypeuk.com/viewtopic.php?p=41715#p41715
The trouble you will have with new vinyl is the centre dash will not match the vinyl grain pattern on the fascia's either side so you will need to do all three. Alternative is to try and buy an original dash panel but only an E-Type one will fit so beware of buying a Mk2 one.
It sounds as if Aldridge sent you the wrong type of vinyl. Ask them for 'leatherette for the dash panel' which is very thin'. I think I have a roll of it somewhere - PM me your address and I will send you enough for all three panels, if I can find it.