Virtually all baptisms pre 1813 in Beds have been extracted onto the IGI. If the baptism isn't on the IGI then you're unlikely to find it in a transcript, from which most of Beds entries on the IGI were taken. There was no birth record at that time (not until 1837), just the baptism.
It's a big gap to fill from the 1550s to 1692.
The first Bitchiner burial that I can find in Kempston wasn't until 1720, implying to me that they came from elsewhere. Look also for spelling variations eg bichinol
What you found on the IGI in 1698 was a member submission of the type that's not worth the paper it's written on. Someone found Robert's marriage in Kempston in 1723, and it being a well known fact in the LDS church that ALL males marry at age 25, has deducted 25 from the marriage year and assumed he was born in the same parish as he was married in, hence Robert Bichener born 1698 in Kempton. Pure fiction. There was more than one Bitchener baptising children in Kempston from 1719 onwards. I'm sure they were connected. As Robert is a slightly less common name than William I'd look laterally and concentrate on finding Robert, and then see if he had a brother.
William was a tailor by the way, in case you didn't know, and Robert was a blacksmith.
David