Any suggestions?

Hi All,
Long time no post. I've mentioned this issue before, but now I'm determined to rectify it once and for all, one way or another. Or the car has to go.
As it's MOT month again and given the previous two have been clean apart from the tester really, really struggling to get it through on emissions In expecting the same.
What I've done: changed the cat converter for on from 'Bitz, changed both Lambda sensors, taken the TF for a 'spirited' run just prior to drop off for MOT. I have spoken to Retro Sports in Bailiff Bridge and they don't rate the 'cheaper' converters and they proposed a £350 replacement, but I'm loath to spend that for the possibility of no resolution. Nothing has made any difference to the difficulty the tester has on getting it through. He might even be allowing leeway given that he sorts my Freelander and the wife's Mini also. Coupled with the above, I'm still getting the MIL every 100-150 miles, usually with no code but if there is a code it's always P0420 - post cat sensor.
So suggestions welcomed.
Regards,
M.
 

Same sort of issue with my F every MOT, so off comes the Cat and on goes the spare one then it usually goes through. Both were/are new Cats from the same brand/supplier, so I don't understand what the real issue is.
Good luck with yours.

Personally id throw down the gauntlet with Retro Sports - agree to pay for their cat but only on the basis they will take it back for a full refund if the car fails the MOT retest.

Thanks for your replies.
D4:
I'd actually thought about a similar thing but with a straight through tube in place of the cat, thereby 'saving' the cat for the MOT eventually. Then back to the tube. Not legal I know but difficult to spot without getting underneath. But your idea at least stays legal. Maybe it's just a quality control issue and another cat from 'Bitz would solve it like you have.

Notanumber:
I could try that but me and my MG are completely unknown to them, so my instinct is they'll not risk it. But no ask no get.
On a semi- serious note the BGT's are starting to look really tempting!
Cheers,
M.

Where do you live?  You could borrow the cat. off my TF if it's convenient.  Exhaust and cat are original, with low miles, so it SHOULD be OK. 
Problem with bits these days are the same for most old cars. Cheap far eastern junk.  MG are probably better off than most makes.
Wouldn't a bypass pipe upset the ECU??

I went through this farce almost every MOT. It is almost certainly the cheap cat causing the problem. As my MOT tester was a mate, he showed me how the cat would suddenly start working when it achieved a certain temperature.(Not quite glowing red, 😂)Below that, it was inert. He said the cheap cats contain enough reactive material to get through one test, after that it was always problematic.
 
I was fortunate that I managed to snag a new OEM cat for about £100. I can't recall who the supplier was but it was one of the usual suspects who happened to find a batch of OEM cats.
I fitted that, and the car flew through the emissions test every MOT after that; no Italian tune up, even, not really warmed up.

So maybe I should keep my original cat. for MoTs?   

Vince:
I can’t speak about TFs, but I know a bunch of Defender Td5s without working cats and central silencers that do not seem to have any problem with their ECUs. 

Many years ago I owned a Defender Tdci without  them and same result.

I do not know the reason why, though.

Hello Vince,
Thanks for the offer.
I'm in N.Yorkshire, Tadcaster/York area.
Cheers,
M.

Sorry Max.  Down south, east of London. 

It has started me thinking about whether it would be a smart idea to get a daily cat and keep the original for Mots!  

I have to agree with David on this, the OEM cats are far superior to the after market rubbish. 
Can you find a original one and just use it for the m.o.t?
 

Its almost worth a group buy for one original cat which we could each use to sail through an MOT and then post on to the next person