I am surprised no one tried cycling the likely routes at night QC, I suspect they would not have got very far before landing face down somewhere!
The Moon is not very bright, and there is a big difference between full Moon and the other phases. I think a super Moon at high altitude is something like .3 Lux and a full moon generally 0.1 lux but it depends greatly on the local weather conditions. On the 7th the Moon phase was a bit over 60% I don't think anyone could cycle with this? It always seems to be full Moon when I am trying to use my telescope!
If a cyclist was seen that night at around 3 to 4ish JB is done for, I just cannot see him cycling apart from the difficultly of doing it with no lights? Adam thinks it's a piece of cake oh well.
If the bike was seen at Bourtree Cottage before the incident, then in my view it weakens the plausibility of a scenario in which Jeremy uses the bike to go to the farmhouse. Within the scope of a Jeremy scenario, I agree with the police that if Jeremy is going to use the bike, it will be on the way back only. Having realised he needs to be at Bourtree Cottage in double quick order, he sees the bike outside in the kitchen yard and takes it.
As to why it would be at Bourtree Cottage, there may be an innocent explanation, such as the one Jeremy offered, and it could be that Julie neither denied nor confirmed this because she and the police were worried it could point to implicating her. Instead, the point is passed over in silence, Julie merely stating that she saw the bike at the cottage on the 4th., and Jeremy had previously mentioned using the bike in a murder scheme.
Adam is reading those two statements of Julie's together to conclude that Jeremy must have used the bike to reach the farmhouse. Personally I think it may have occurred to him and then he dismissed the idea, realising that a ladies sit-up-and-beg bicycle would be an absurd impediment to him. He changed his mind for the return leg only because he had no choice.