Vauxhall Astra oil filter leaking

Entire filter housing can come off,
And there's seals between the housing and block.
Could be those seals are the culprit.

" First things first, make sure that the oil leak is not from some dozy idiot putting the oil seal O ring that came with the filter around the top lip of the filter screw cap rather than in the recess halfway down the threads!!! That has been done so many times it is unbelievable!!! If this is not the case and you have wiped all the oil away from the housing area and it is indeed leaking then you do need to replace the gasket."

This shows the three bolts that allow the unit to be removed to replace gasket / seal if needed ( same as Corsa )

http://ecat24.com/opel/catalog/part/1/16/185/3386/35644,35654/

Thank you 1/4 Drive and Whittinghame for your prompt replies. When I fitted the new filter the old oil seal ring was under the top lip so I replaced it that way. I will have it out and do it as you stated. I'll keep you informed. Regards MRMOBY

Hello again. I removed the oil filter top and checked to see if there was a groove for the ring halfway down but there isn't. the actual filter casing was dry, the oil being at the bottom. So the housing will have to come off. How big a job is this?.....Does the radiator, exhaust pipe, etc need to come off?
Cheers MRMOBY