Hi macaoidh!
Hmm, it's a difficult one but I compared with many in persons from
AutographWorld and I'm leaning towards
authentic.

Here's
in person examples from AutographWorld:


It's one of the rare more "neat" Newman autographs I've seen, usually it would look like this:


To conclude, I think it's authentic.

Remember,
definitly not all autographs on eBay is fake - there's lots of authentic ones too but you really have to know what to look for, then it will be easy to spot fakes from real ones.
Good luck,
zanne