Ok I enlisted the help of some other research fiends. Here is what they wrote back.
". . .The Messines (7-14 Jun 17) attack was carried out by British Second Army. The Corps sqn's in direct support of the attack were: 21 Sqn, 6 sqn, 53 sqn & 42 Sqn backed up by 21 Sqn & 7 Sqn (RE8's & BE2's)
Second Army offensive asset was 11 Wing RFC - 20 Sqn (FE2d), 45 Sqn (Sop-2), 1 Sqn (Nieup), 41 Sqn (FE8 ) 7 46 Sqn (Pups). They were reinforced by 1 Naval & 10 Naval both on Sop Tripes.
In addition 9 Wing RFC provided: 19 Sqn (SPAD), 56 Sqn (SE5), 66 Sqn (Pup), 27 Sqn (G100), 55 Sqn (DH4) & 70 Sqn (Spo-2).
23 Sqn (SPAD) & 40 Sqn (Nieup) from First Army also flew missions over battle area.
On the opposite side was German 4 ARMEE who up to June only had five Jastas - MFJ 1, Js 8, Js 18, Js 28 (though nominally 6 armee) and Ks 8. Also 10 recon - Fa3, FA 6, FA 19, FA 33, FAA 213, FAA 221, FAA 250, FAA 294, MFA I & MFA II.
In the course of the battle they were reinforced by: Js 20, Js 4, Js 10, Js 6, Js 7, Js 11 & Js 26 (in that order) In addition Js 27, Js 30 & Js 33 from 6 Armee also flews patrols over battle area.
2-seater reinforcements look to have included: SS 4, SS 11, SS 17, SS 19, SS 21, SS 23 & SS 30, FA 7, FA 8, FA 26, FA 40, FA 45, FAA 202, FAA 204, FAA 231 and FAA 224.
By end of month Js 27, Js 36, Js 17 & Js 24 had all been attached to German 4 Armee.
As Gen von Hoepner recorded in regards the 1917 Flanders offensives: 'Because of their number and their sporting audacity, the English continued to be our most dangerous adversaries and, as before, the major part of the German air strength was concentrated against them. . ."
See me for abbreviation explanations.