Sounds familiar, unfortunately.
I own a Bosch Car Service Centre and do the work for Trading Standards in this area, including verifying problems & work done by other garages (even fix their personal cars).
You definitely need to contact Trading Standards & the finance company.
The dealer IS at fault for not initiating the warranty, but, with my experience with warranty companies is that they won't be interested in such a fault so soon after sale as they will argue it was there at the time of sale (whether it was or not).
The car probably "Wasn't fit for purpose" as spending even 2 or 3 thousand pounds on a car it should last more than 3.5 weeks.
Unfortunately… you have admitted driving it around with the EML on, and indeed a lambda fault (i.e. fuelling fault, which may not be the lambda sensor but indeed an uncontrollable rich condition) this in itself may not help your side as an over fuelling engine dilutes its oil and may be used as an excuse for mechanical wear (oil testing will confirm this).
The long and the short of it is you (or someone) WILL have to authorise an inspection to verify the fault AND the reason for it occurring.
And you should stop driving it, subsequent damage will almost certainly NOT be covered.
Previously owned Alfas - Alfasuds (first car was a Ti Cloverleaf) 33's, 75's, GTV's (Bertone & 916), Giulietta (1983), 164's, 145's, 155's, 156's, 147's, now Brera!