I was on Fixed Wing Test Sqn for 88-90 and ZD 320 appears in my log book from Apr 88 to Jul 89. ZD 319 from Sep 88 until Nov 90. ZD 318 only appeared, as far as I was concerned, as a GR7 26 Apr 90 (first RAF GR7 flight) to Sep 90. My recollections are that 319/320 came and went between Boscombe, Dunsfold and West Freugh depending on what trial they were supporting at the time.
This is ZD319 on the first grass strip trials at Boscombe 21 Sep 88. Whilst I'm in the aircraft, I don't have the copyright, which I guess lies with the MoD
ZD320 on the way to Aberporth for AIM9L guided firing 9 Dec 88. Same as before - the copyright probably belongs to the MoD.
As far as pictures of ZD318, there is one on Airliners.net from a Malcolm Clarke showing the aircraft at Boscombe's Open Day on the 9 Jun 90.
Hope that's some use to you.