Hi Alan,
It might also be worth looking in Oxfordshire, the OFHS do have Harwell's parish records amongst their parish records CDs so I guess Harwell was at one time in Oxfordshire. I've checked my collection of OFHS CDs but unfortunately Harwell isn't amongst them, my ancestors were based more to the East of the county.
In case you want to buy the CD that it's on their reference number is OXF-WAN02. This CD also includes the parishes of Ardington, Blewbury, Denchworth, East Challow, East Hendred, Goosey, Upton and West Hendred. I don't know that area too well so don't know if any of these are contiguous to Harwell.
Just checked, East Hendred is contiguous. The other contiguous parishes are Milton (AB01), Chilton (WAN01), W,Hagbourne (WAL02?) and Didcot (WAL02).
OFHS have a map of all the parishes which might help you spread out from Harwell.