I followed the link provided by Natbag which took me to the Check 2 Fly website Most of it is quite straight forward however there are a number of issues.

1. Although there are 2 options for taking the test (Drive-in, or at gate 3 Terminal 3), only the Drive-in is available at the moment
2. The drive-in (not to be confused with drive-through) looks like it requires parking in the car park and following the signs to the arrivals area of terminal 3. - I've not been yet so somewhat unsure at the moment.
3. Of the 2 options for results (4hrs or 14hrs) only the 14 hours is available. Also, it should be read as a minimum of 14 hours for the results. it could be up to 3 days.
4. Supposedly you can only take this test 72 hours prior to the flight but I'm flying on Sunday and the only option given was to take the test on Thursday - about 67 hours before departure.
5. I selected to take the test at 14:30, but when I log back into the site it tells me my next appointment is at 04:30pm - I'll turn up at 14:15 and see what happens.
6. The site informs you that there is a downloadable app both for Android and Apple but neither links work and I couldn't find anything at the Google Play Store. Apparently, it doesn't exist yet.

Update 18 Nov

• By this evening I'd still not received any email confirmation of my scheduled appointment for tomorrow, so..
• I went back to the site, logged-in and found to my dismay that there are no appointments listed in my profile (so even the one for 04:30pm wasn't there let alone the one I actually booked and payed for at 14:30)
• I started the process again and found that the 14:30 slot was available again so booked it, gave my CC details, and "Transaction Failed".
• I started again, The 14:30 was now taken (presumably by me already) so I booked 14:20, gave different CC details this time and the page then hung in "processing...." mode. The transaction pinged on my CC phone app as having been successful yet the web page still just hung until eventually, it switched to an error code.
• At this point, my frustration was mounting but still persevered. I decided to try it on Microsoft Edge instead of Chrome. Logged in, again no appointments appearing in my profile so yet again I went through the process of booking another one. the 14:20 was now taken so selected yet another slot. gave the CC details and voila, it worked. The transaction was successful and the appointment is listed in my profile. Now for the next trick.
• To the right of the page, it states you need to come armed with your passport, printed flight ticket, and printed QR code - where to find these? No email was sent and there are no instructions on the site so pay careful attention -
• Click on My Appointments. A list of scheduled appointments appears to the right. Next to each one is a subtle ">" arrow. Click on it and your QR code appears with the option to print it or send to email.

Now I have no idea whether tomorrow I'll have 1,2,3 or 4 appointments, nor whether I'll be charged for them all but I'll report back with details of the experience.

Update 20 Nov

• The testing area is in the Terminal 3 Arrivals Hall. They've cordoned off a section between the windows and the escalators running from Entrance 03 to 01.
• Starting at Entrance 03 you stand in line (yes there was a line) where there seems to be a mix of people with appointments and those without. Every so often someone would walk along the line searching for those with appointments.
• Those with appointments are 'fast-tracked' to the first stage of the process which involves showing your passport and QR code for scanning. They then issue you with a numbered ticket for stage 2 and a barcode sticker for your test tube.
• You then move to stage 2 where you stand in line again waiting your turn to be called to one of 5 or 6 testing booths. The test takes all of 3 minutes and you're done.
• This morning I found a text message informing me of the results and a few hours later a recorded phone message with same. The full printable pdf version is in the personal area of the website.

I spoke with the person in charge there and informed him of the various areas where I thought the process could be improved to make it more user-friendly. He responded favorably and asked that I send him details by email. Whether or not multiple charges for the non-appointments I mentioned a few days ago are credited to my CC remains to be seen.