Thanks to everyone for their help! I've now had my 2nd smoke on the offset and wanted to share some thoughts and ask some more questions.
Firstly, some pics:
https://imgur.com/5wr0tnChttps://imgur.com/9DEvzS6https://imgur.com/aFy7pG7https://imgur.com/EACiSe2As you can see, it turned out amazing - SO THANK YOU!
I ran the closed box method this time successfully too (WOHOO!). It's only a 16x16 firebox so not that big meaning I can't use huge splits. I managed to keep the temp consistently at between 250 and 300 (temp monitor on the grill was slightly passed halfway towards the firebox so meant the cooking area temp would be between 225 and 250 most the time) so this was perfect.
THE PROBLEM: I did struggle for a period of 90 minutes twice, where there weren't enough hot coals to keep the log alight fully, meaning the temp dropped all the way down to 190 and it was a real struggle to get it back up..... This led to some major frustration.
The first time was right at the start - getting it hot enough. I used a charcoal chimney full of lump. Set that alight, waited 15 mins for the flames to all die down and poured them in. I kind of stacked them towards teh backside of the firebox rather than spacing them out (which may have been an error it meant a big hot spot was towards the back corner rather than more central for the entire day!). Then stuck a couple of splits on which set alight but still didn't get up to temp... It took quite a few splits in the end (maybe 5 or 6) before it started getting hot enough.
Once it was up to temp though, I was rolling pre heated splits on 1 or 2 at a time and they'd set nicely alight.
Then a few hours later, it dropped back down to 190ish. It just felt like the coalbed would disintegrate, but the logs on fire wouldn't be ready to be broken up into hot coals and that would leave a kind of patch where everything would die down and it took a lot of time stoking and adding more logs before it would replenish.
I often found that the logs I'd put on would be on fire but the ends may not be, so I'd have to do a lot of turning it around, moving the coals underneath and trying to keep the whole thing on fire. This meant a LOT Of time spent moving logs around in the fire itself which I don't think is needed if the coal bed is properly design - is that right?
Anyway, your advice and thoughts based on the above would be appreciated. The majority of the cook went great though - I did have to use 2 splits on fire most of the time to keep the temp up and keep that coal bed developing, so wasn't able to rely on just putting one split on at a time which was unfortunate. I went through maybe a couple of nets of oak for a 12 hour cook which seems like a lot.
Thank you again though - even with all of that, it was a great day, a great smoke and the food was INCREDIBLE!