Your timings are way out from PC Wilkinson's evidence: http://www.jeremy-bamber.co.uk/bicycle
The bike is a big piece of circumstantial evidence. So big that the judge included it in his summing up to suggest Bamber's guilt.
However it is not one of the huge amounts of pieces of incriminating evidence David has said (without elaboration) was created by 'forgery & perjury'.
Bamber himself said he brought June's bike over to his cottage just before the massacre. So there is no dispute.
The prosecution suggested Bamber cycled to & from WHF. Backed up by Julie's WS which said he planned to cycle. The police, DPP, jury & appeal courts obviously agreed this was possible.
Or people thought Bamber walked & it was just a coincidence he took June's bike just before the massacre.
Either way a massacre was possible.