Console yourself with the thought that there is no tomato on earth that will never ripen given long enough or the right treatment!
There are three factors which are relevant to the ripening of the fruit: 1 Size of fruit. 2 Warmth and heat 3 The presence of 'ripening gases' like ethylene.

I hope you are keeping your greenhouse closed at night and allowing temperatures to remain as high as possible during the day. As we are now in September day time temperatures are lower and nights cold, so ripening can be delayed. Have you tried automatic vents which close when temperatures fall below a certain level?
At the worst, you can harvest mature fruit while still green and wrap in tissue and place in a dark, warm place, where they will ripen. Alternatively place them in a bowl with ripening bananas where the ethylene will hasten the ripening process.

Others will disagree but I would suggest that tomato problems this year cannot be entirely due to the climate.
I have had my best crop ever from a mid March sowing, albeit about 2 weeks behind normal.
I would suggest compost used, how they are grown and varieties are the biggest problems, in that order.
Decent compost...not garden centre reduced peat rubbish, regular watering and regular feeding are the priorities. The rest will follow.

You do need warmth to ripen tomatoes, but excesses of heat can delay it....as long as your greenhouse can stay above 60'f during the day, and not go above 80'f for too long, they will ripen.

Ours are only just ripening now.

Mine have started too Bulba, about six weeks late....but I think we'll get there in the end, patience......

I have been picking mine in the poly tunnel for about a week and all is good so far considering the allotment has been hit very badly with blight most have lost there spuds and the toms are getting as bad.

Mine began t ripen when I draped the stems with banana skins - started to take effect after only a couple of days after weeks of waiting. Worth a try - you might be surprised.

I've not tried it in the gh but it certainly works in the fruit bowl........on melons too. they ripen much quicker

I have to agree with Scrumpy, good variety of tomato and plenty of water and feed. Grew 'Shirley' again for the sixth year. It is good greenhouse variety with not too much foliage so allows plenty of light to the fruit and a good virus resistance. Started off in a propagator and grew on without a check and kept on the ball with regular watering and feed. Have been picking for the last three weeks, best year yet with 'Shirley'.

I,m usually picking the small ones the first week in july this year was mid august, but we are on a hill open to the north and east which doesn't help!