The unit code on ED559 was JA-D. I've seen that 100 Squadron at Waltham adopted HW. I've not been able to find out when the change was. I also saw on one site that JA was the unit code for 1652 HCU (Heavy Conversion Unit). So, to hopefully get some answers I posted a query on a forum.
UPDATE - I had a number of replies and added the detail about the JA code to the 100 Squadron page.