Thanks for your input guys! I've done a quick calibration on it and I think the blue is the OIII line at 500.7nm rather than the H-beta at 486.1, assuming that the red line is H-alpha at 656.3nm.
I didn't notice the other faint one, thanks Merlin!
It appears to me to come from NGC 1910, another bright nebulous area in the LMC, but I'd would welcome a correction on that.
Interesting really, presumably if you zoomed in on the Tarantula Neb, the spectrum would show as two Tarantula images (three if the H-beta 'line' showed), equivalent of two or three narrowband images for the price of one! Might try it if I get time.
Cheers -
Rob