I have a 2014 Focus 1.6tdci. Over the last couple of weeks it had a sudden loss of power (similar to what limp mode feels like) on several occasions, lasting roughly 5 minutes on each occasion. There is no engine warning light coming on and I have found no fault codes with my £9.99 eBay diagnostic machine. Roughly when this problem started the engine fan started running regularly after the engine was switched off which is unusual for this car. 
I intend to take it to the garage but it would be good to be armed with a bit of knowledge before I go. Any information or help will be very much appreciated. 

Usually it's the DPF failing to regenerate that causes the fan to run on after engine shut-off.  Though due to the extreme heat we're having currently, it could also be caused by high coolant temps.  I assume the coolant temp gauge isn't reading too high and you've checked the coolant level?

If you have no warning lights, it probably just needs a good run to get the DPF fully regenerated.  

Forscan is better than a generic ebay tool or, in general, a garage tool.  Costs about £20 for the ELM adapter and the program is free for a laptop.