Can anyone suggest a appropriate feed for a rather pathetic Olive Tree ?

I'd ask first how long its been in the same pot? If you haven't repotted it for some time, its probably potbound and that might be why its looking so pathetic.
Bit late to feed it now, but you could give it a shot of Miracle Gro general purpose - but not after the end of this month (preferably not after this weekend, really). Otherwise, if it needs it, repot.