will do!

DONT GET FIVE LITRES - IT ISNT ENOUGH. The box holds 7 and a bit so you wont even be touching the sides.

Get 20 litres and use half - use the other half in 18 months

Those 20l kegs are the correctly sized "dream buy". I wish we had 'em here.
However, the OP is doing his first drain & fill and <4l will drain so a 4, 5 or 6-pack purchase is OK. Likely he will need additional drain & fills but we don't know how he'll react to the experience of doing this job.
Even though a drain & fill is quite easy, many find under-car tasks horrid (or have a bad experience like spilling) and will never do it again.

Remember OP, you don't want a small plastic basin suitable for washing a new born kitten. You need a larger plastic pan; one with a pouring spout.
Also, do not buy one of those odd drain jugs where you let the draining fluid into the side of the jug-then stand it up like a suitcase.
They splash to no end and the big, screw in plastic disc (which seals the open side) are complete garbage.

Decide for yourself. Skip the shipping and reduce the drama and get your first d&f done with a 4l purchase. If the fluid is black and bad (likely) then your subsequent drain & fills will eat up the keg.

Don't saddle a first timer with a 20l keg.

I've gone through many 12-qt. cases and >20 gallon jugs of several ATFs and am very glad I no longer warehouse the stuff.
Companies here sell appropriately sized kits for drain & fills. It's what I now use on my Volvo and won't need for another 20,000+ miles.
Today I've one, sealed 6 quart case of MerconV for my truck and 4 qts. of Mazda FZ for the Mrs' CX-5.

I'm getting older and the thought of my widow giving my ATF stash away or selling it to some scraggly, opportunistic piece of $#!* for pennies on the dollar irks me.

+1 all very useful info but I would add to that, I use a bucket for draining

plus points being if you jack it to the right height the bucket is basically right up against the drain hole and the liquid doesn't blow around in the wind so much
also less chance of a spill

I then pour carefully from bucket into measuring jug (3 or more pours) and then into the oil disposal container (dispose responsibly!)

it's a nice easy way to both measure the fluid and also see it nice and easily for colour and if there's any bits floating. one would hope not

when it comes to filling up, pouring from a 20l keg into the measuring jug will be fun, you might want a 2nd pair of hands and an old towel laid out

thanks all
I have bought 6 litres as people have said that for the first sump dump about 4 litres is released. It came as 6 litre bottles and was about £28
I still need to get a replacement washer for the sump bolt thing (can this be bought online?) and also some way of putting the fluid into the dipstick tube so some sort of tubing (anyone know the correct size or is there a funnel I can buy that is good for the job?)

If you haven't come across it already

https://www.volvoforums.org.uk/showthread.php?t=160516

Provides a wealth of tips on 'how-to'.

I'd seriously recommend you clean around the filler plug and make sure it will move before you actually try to change the oil - its probably not been moved for 17 years so its likely to be a bit reluctant! When I did my gearbox oil change the first time the only place I found for seals and washers was Volvo

I've just re-read your question and realised you don't have a geartronic box - in which case it's a lot easier. You will just need appropriately sized tubing (too small and it takes forever - too large and you can cause an 'air-lock' in the dipstick tube. Attach the tubing to the funnel and support it so you can refill it without holding - otherwise you will find youself without enough hands.

Best of luck

John

thanks - what do you mean by filler plug? I thought it was only fill-able via the tube the dipstick sits in?

See previous, edited post!

I do have a geartronic box - the 5 speed one that's supposed to be 'sealed for life'
it has a dipstick in the engine bay