Michael Bell writes
I keep log of my health and looking back through it I only realised
last year that I get a massive attack of sneezing and runny nose and
feeling generally unwell each year about the 3rd weekend of May. My
doctor recognised this as hay fever and prescribed Cetirizine which
cured it. (Dose this prove the diagnosis?)

Cetirizine is an antihistamine, which as i understand it works to
counteract the histamine which is the body's signal to let water near to
the skin surface to counteract something which the body perceives to be
a foreign intruder or a wound. It's used for all sorts of allergic and
auto-immune skin reactions, as well as for hay fever. I think it is also
used, like other antihistamines, to combat car sickness and other
similar types of nausea. Don't know whether this means the diagnosis is
proved ;-)

Yesterday evening I got the first attack of what felt like the same,
but it has gone this morning. What plants and their pollen are the
possible causes?

Could be almost anything but grass pollen is a common one early in the
season.
--
Kay