Hi Mark,
Hmm - I have a good amount of room. I recall it was easier to do it from underneath the car as there was more space to hold the canister in place with one hand whilst tightening the bolt with a ratcheting ring spanner/wrench in the other.
I ended up using my old seal: Oil Canister Seal 6883 Dimensions (XK150, E-Type, Mk2) - #5 by clivejer
Regards,
Clive.