A maggot found in an orange imported to the UK from South Africa
No interactions present.
I used the scientific name from here:-
With a fly this distinctive it took a few minutes to get an image of the lava.
It did look right though.
Thanks for the comments. Having looked at some pictures of larvae I think that RHoman is right. The larva and fruit damage here: http://www.invasive.org/browse/subinfo.cfm?sub=4989
matches what I found very closely. The fruit-fly larva here: http://entnemdept.ufl.edu/creatures/fruit/medfly21.htm
is creamy white rather than orange and doesn't have obvious head markings.
Lat/Lng: 51.3, -0.4
OS grid ref: TQ1663