I'm trying to find an oil filter for my 1.9 Mjet but having difficulty, however a Holden (Vauxhall) dealer can supply a 1.9 Astra one. They are located a distance away from me so I cannot compare it to mine.

Are they interchangable?

Probably... same engine really. Over here pretty much any half decent Car Parts store stocks them, my local Indy repair shop use Mahle equivalents for oil & air filters.

http://www.mahle-aftermarket.com/C12570B3006C0D49/CurrentBaseLink/W26MNK9W508STULEN