DPF problems? Or something else?

Hi,

This is my first post, so hope I've given enough detail below. Any info gratefully received. 

Bought a 61 plate S-Max Titanium Sport 2.2 last October. Was running fine till March this year when it starred making a rattling noise. Took to local garage (non-Ford and have used in the past for other cars with no issues) and needed a new exhaust (or part of at least) as it was falling off. 

Ever since, we've had issues with smoke from the exhaust then Engine Malfunction warnings and engine light on the dashboard. It's been back to the garage 6 times in 3 months to get fixed, but every time we get it back, the same thing happens with about 10 days. Initially they said fault was a historic code triggered by service after replacing the exhaust, but it kept happening. They've sent it to electrical specialists, and the local Ford dealer who apparently supplied the after market filter for the replacement exhaust, it's had a regen and new air flow sensors fitted but is still gubbed. This time they're sending it to a fuel injection specialist garage as they don't know what to do with it.

It ran fine until the new exhaust/filter was fitted, so I'm thinking that's the main cause of this happening, I've no codes to go on but could it be something else triggered by new exhaust being fitted?  The smoke appears mostly when accelerating between 2nd & 3rd gears, especially if on a slight incline but not always. Once malfunction warning on, will be ok once car stopped and restarted but the engine light comes on, then a few journeys later the malfunction happens again and then every journey - it runs in limp mode and is rubbish to drive. There's never a repeat of the smoke after the initial burst.

Anyone had anything similar or suggestions as to cause/solution?  Thanks.