A bit of overtime helps Lookout 
Well Jeremy did advertise the murders ahead of time, Rob. It would have been a bit rude of him not to turn up.
Anyway, Ainsley says 15 minutes by bike using the Brook House Farm route (return only). DI Wilkinson completed the test ride in 16 minutes, but that was by day.
It's worth emphasising that at roughly 2.30 a.m. on the 7th., you would probably just be coming into astronomical twilight, which means it would still be dark, albeit with the benefit of moonlight since it was between a full and three-quarter moon on 7th. August 1985, and I would calculate roughly 65% lunar illumination.
In plain language: you'd still need lights on your bike when going along, unless you don't mind crashing into potholes, pedestrians, anything. And it would take him longer than 15 or 16 minutes, for a few different reasons:
(i). if he is riding without lights, then please at least double the time estimate;
(ii). it's dark anyway, so even with lights, it will take longer along farm tracks;
(iii). it's been raining, thundering and lightning, just before this happened, so the tracks will be muddy;
(iv). it may take him some time to get into Goldhanger and reach the cottage itself, as he must be cautious and not be seen. Even if he is disguised, a probable retroactive identification is still possible. "Oh yes, I saw somebody on a ladies bike at that time, cycling down Maldon Road he was. Yes, his face was covered, dressed all in black, no lights on, and he stooped as if he not to want to be seen. Thought it was strange."