My DS1 took the test at Tommies last year.
I bought him an analogue watch to wear on the day, following the same advice on this site that you've read re analogues being better than digital. (Not really sure why...?) Anyhow, I then stressed about the watch battery running out on the day and whether it was set to the right time etc.
DS1 says there was a clock in the room he was in and the invigilator gave them time warnings (10 mins and 1 min to go, I think) so in the event he didn't look at his watch at all!
You definitely shouldn't send the children with pens, as the papers are marked by a computer which reads pencil marks apparently.
Tommies sent us a letter of instructions a couple of weeks before the test and the boys were told not to bring their own pencils, as they would be provided. I seem to remember those sitting at Pate's did have to bring them though. I'm sure it'll all be laid out in the letter you get. Do allow plenty of time to get to the venue and park on the day, as there will be lots of people trying to do the same thing.
As for a snack, I sent my boy with a banana and a small chocolate bar in an attempt to give him an energy boost mid-way.
No idea what a rectangular pencil is! I've seen triangular ones to help children with their grip when they first start to write.....but rectangular? - not a clue. I'm sure it's not obligatory anyway.
Good luck to all of you with DCs sitting the test shortly. I know it's a really stressful time.