Thanks for posting their reply Dave

At least people will know now Not to bother getting a E-visa. Especially given that some were lucky getting through - some weren't, it's probably best to avoid it altogether (even if flying with THY and/or for business).
As you say, it doesn't explain why they are accepting payments from everybody who applies regardless of who they are travelling with or what they are travelling for!
If they are only applying this facility to their own National Airline (at this time) why the heck doesn't the E-visa website make that Clear! No thought (again) to the distress & cost this is causing people arriving in Turkey

. Surely their website can be adjusted to reflect the eligibility of the person travelling!
I hope that they are going to apologise & re-imburse the people they have taken this money from, I wonder how many $20 in E-visa fees that adds up to so far?...........

I have just had this reply from the British Embassy. I asked them if the eVisa was in force and if so why some people are having no problems but others are having to pay again at Dalaman having already paid on-line.
I don't think anyone was aware officially, that it only applies if you fly THY (Turkish Airlines) at the moment. It still doesn't answer why some people have got through and why some haven't.
There is a new e-visa regime for tourists which has been announced by the Turkish Ministry of Foreign Affairs. It is available for "tourism or trade purposes". We have established that it runs alongside the existing system of a £10 sticker/stamp-type visa which can still be acquired at airports for the foreseeable future. For the time being it is only available for foreigners arriving in Turkey on flights of the Turkish Airlines (THY), the country's state-owned air carrier, in line with a protocol signed between the government and THY.
The system is up and running. It is a little more expensive than the paper visa (US$20 for UK nationals paid by credit card). It is apparently easy to use and has been successfully used by UK nationals.