Really need to do a ATF fluid change on my 1.8 automatic.

Is it possible to suck out all the old via the transmission dipstick tube?

I know it should be really down from under but I always like the less sweat and mess options.

You could, but the point of a fluid change is to get rid of as much collected crud as possible, so draining downwards has a much better chance of letting out the bad stuff than trying to suck upwards. The heavier stuff will simply collect in the remaining puddle of fluid, and you'll end up with decades worth of stuff you really don't want left in your gearbox. I imagine the fluid pathways within the auto box will have been carefuly designed so the nasty stuff tends to sink down, so trying to draw it all back up through the dipstick tube runs the risk of bits of unpleasant-ness being dragged back up into the box and maybe getting stuck there. I'd recommend this is worth doing right, and your auto box will be much nicer with clean fluid and without the residue of aeons left in it

So a quick update, I thank FON for the advice it was true, accurate and everyone should follow it.

My ATF was 25 years old, 70 thousand miles traveled and a light brown colour.

Now I did not follow FON advice but went with the suction method. Even on draining you will never get all the ATF out due to the torque converter. The capacity is 4.9 litres on a 1.8 I was able to pullout 2 litres put 2 back, cycled the gearbox and repeated.

Went for a quick drive and gear selection was more positive and on a longer 80 mile drive. Kickdown was instant and lower RPM at 70mph than I had noticed before.

So to sum up, there is the right way and the quick, I was lucky and the quick worked.