453 Sqn Operational Records are in the link below, W.J. Harper first appears on page 5 as being on route from the UK 12/9/41. Harper arrives on page 9 on 2/10/41 and is mentioned throughout to page 28 in Feb ’42. On the next ORB from June ’42 at Drem to Jan ’46 there is no mention of him.
http://recordsearch.naa.gov.au/scripts/Imagine.asp?B=1359053
http://recordsearch.naa.gov.au/SearchNRetrieve/Interface/DetailsReports/ItemDetail.aspx?Barcode=1359195

His combat reports for 18/5/40, 25/5/40,11/8/40,15/8/40 and 8/11/40 are available on line here for £3.50.
http://www.nationalarchives.gov.uk/documentsonline/details-result.asp?Edoc_Id=7738390&queryType=1&resultcount=8

Regards,
Al.