worcester boiler - condensate pipe problem - but no ice?

Hi - we have a worcester boiler which last winter tripped out a couple of times - after googling and finding this website, plus the blurb on the worcester website I deduced this was the frozen condense pipe thing and set about with hot water bottles and kettles accordingly. last winter only happened twice in very cold weather. This winter happened the other day for the first time - hair dryer, hot water bottles and kettles galore but didn't kick back in until the afternoon (when sun was shining on it) have had it on constantly since then but decided to turn off for a few hours during the night last night - oops. now happened again. but the thing is our condense pipe only sticks out about an inch outside the house. I have pushed a foot long skewer down it and there is no hard blockage - although pulling the skewer out brought with it some sludgy water. I have hot water bottles wrapped round the inside part of the pipe but this is inside a warm room - how can the pipe be frozen inside the house and not outside?
There are two bends in the pipe within the first foot after it leaves the boiler then it is straight for about 2 foot (which takes it through the wall of the property and about an inch outside)
Any ideas of how to get it working faster rather than waiting all day?
long term fix - should the pipe be sloping at an angle (as it really isn't)
Could it be that the pipe is just sludgy and that when the weather is cold the sludge gets harder?
Or could it be a completely different problem after all!
TIA for any help or advice!