BALIX,
AIC 97/2004 (Pink 70) says this, which is not really a lot...
BLOOD, PLASMA AND BONE MARROW DONATION - FLYING AND AIR TRAFFIC CONTROL
1 Many aircrew and controllers have expressed the wish to donate blood in support of the National Blood Transfusion Service. Blood donation is invariably a safe, painless and uneventful procedure and the blood volume is restored within a few hours by redistribution of body fluids and intake of soft drinks and beverages. A few individuals feel faint afterwards but this effect is minimised by resting supine for a short time (15-20 minutes).
Donations are not taken from those found to be anaemic.
2 Blood and Plasma Donation
2.1 Aircrew
2.1.1 Aircrew are advised that in order to prevent the very slight risk of post-transfusion faintness or syncope they should refrain from
donating blood or plasma if they are required to fly within twenty four hours.
2.2 Air Traffic Controllers
2.2.1 Air Traffic Controllers are advised to avoid donating blood if they are going on operational duties within a minimum of twelve hours. They can, however, give blood when coming off shift.
3 Bone Marrow Donation
3.1 After bone marrow donation which involves a general anaesthetic, the minimum interval recommended before duty for both
aircrew and controllers is forty eight hours. However, the individual response to this procedure varies and specialist medical opinion
should thus be sought.
Best of luck. I'm on the register as well, had to give a second blood sample a while ago but turned out not to be a good enough match. I still hope that one day I'll be able to help someone.