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I think I may have figured out what the problem is. I decided to make a 2L batch over the weekend to see if hitting the pH levels Ann suggested made any difference. In the process, I discovered that my cultures were preforming very badly because they were past their best by about a year. I have only been using flora danica, geo and PC for camembert recently so that would explain why I have been having this problem.
I have ordered a fresh supply of cultures and will have another go later this week and post the results up here. This has been an insightful learning experience   
I also learned quite a bit about my pH meter so my measurements should be more accurate from now on. The batch of camembert is probably not camembert anymore as it expelled far too much whey while waiting for the pH to drop, but it might turn out edible.

I am still going to look into the right amount of CaCl (thanks to those who have brought it to my attention here) as it might be a contributing factor to the slight bitterness I have been having in other cheeses.

Andrew, you may have been right that there may have been some b.linens in the previous failed batches. The orange tinge wasn't particularly bright, but it was there. However, I have given up on that batch being edible at all. Tried some on Friday and it was not very pleasant and only getting worse.

Just an update on the latest batches. I am pretty sure now that the previous failures were due to the correct acidity not being reached because the cultures were getting old.
I currently have 2 small batches of camembert aging, made a week apart. The first was made using the old cultures. This one ended up with a very dry curd because I let it sit to acidify after cutting and stirring the curd. It was during this make I realised that the age of the culture was leading to slower acidification than assumed by the recipe. It seems to have turned into a decent looking cheese, even if it is a bit misshapen (which you can't really tell from the first picture).
The second batch was made with a fresh batch of cultures. When I added the rennet the pH was 6.53 and it was 6.51 after cutting and stirring for 5 mins. After 24 hrs draining in the moulds it was down to 4.76. At the moment, it has been aging for about 2 weeks, has a good layer of geo and the P.can is starting to show up (see second picture).
I am going to try one of the misshapen batch this weekend and, with luck, the second will be ready the following week.
Thanks to everyone who responded here. You comments have helped me learn quite a few lessons about acidity in cheese making.

Wow!  That geo is amazing!  So brainy looking :-)  AC4U!