How much should you expect haemoglobin (g/l) levels to fall after blood donation on blood tests? Does it vary greatly?

Somewhat. Depends on your body stature-height weight, etc. If generally in goof health and a normal adult body weight, your HGB may drop a few points. For example 14 --> 12/13, and if healthy, and with proper diet, you will regain your levels in 3 months. Very small in stature, thin people, may donate a pint/unit of blood, and feel much weaker and drop more points. Best wishes.
Yes. There is a significant variability in how much the hemoglobin drops with blood donation. Donation of 1 unit which is approximately 250ml (give and take 50ml) often decrease hemoglobin by 2-3gm/dl (20-30gm/l). In young individuals, the bone is active and often rebuild the lost blood rapidly. Iron and folic acid/vitamin supplementation may be useful to help the body re-build up the lost blood.
1 unit = 1 gm Hgb. "mostly." if a moderate or larger sized adult, that is the rough place holder to use, and is how we estimate the transfusion requirement for someone who is anemic. So if you were 15.5, post donation, you should be about 14.5. If you had a "double red" donation (uses apheresis to remove red cells and return other components), it takes two units from you. Thanks for donating!