Request, via your ISP that they run a CIDT test, see what that comes back with.
To be honest, there is no 'stuck in the middle' about it, if the problem is with your broadband, then it's down to your ISP to sort this, end of.
I'd guess that the fault is in the exchange also, as this is the only thing to have changed with your service. Would be surprised if it were the jumpering, as suggested. I'd be plumping for an issue with the SMPF port itself (the bit that 'makes' the DSL signal).
Your new ISP should be forced, by you, to raise an LLU SFI2 visit. They will threaten you with charges, but you sound like you have covered all the equipment/internals, so charges cannot be passed on by the ISP.