Not many rules really, this board being aimed at technical issues, it shouldn't fall foul (hopefully) of some of the more personal issues that can affect forums.
Rule 1 - Is that you need to think very carefully before posting anything technical or asking anything technical relating to the security system of the car - See 'Security Issues' sticky for more info.
Rule 2 - We (MGF Register) do not support copyright infringement and therefore references to CD ROM versions of the workshop manual (for instance) should not be posted on the forum (don't want to get into trouble, we'd rather sell you a pukka hard copy through our Regalia shop anyway!)
Because advice is honestly and freely given in this technical section, much of it will be amateur experienced based. Therefore we are pleased that we have a professional MG trained mechanic as a moderator. Steve Tyler who owns and operates British Legends in Southend will visit regularly with his knowledge and assistance.
I purchased one recently on ebay from MGManialtd (Glen Sprigg), which was the genuine MG part. £35 + postage Changing them can be a pain. It all depends how easily the old one can be removed. I can be easier to remove that section of exhaust and remove the old sensor once the exhuast is off the car. Personally I would get a garage to do it, but then I'm lucky enough to have Techspeed on my doorstep.
Lambda sensors always seem to weld themselves in their threads in the down pipe. You're more likely to break your lambda removing tool than you are to actually getting the darn thing out. I suspect a bit of local heat, applied with an oxyacetylene torch or similar is the way to go...
You're going to absolutely love this job. I recommend industrial quantities of tea and a lorry load of jaffa cakes. And more than likely more than one Lambda removing socket. And ideally a heat source.
Let us know how you get on, and what the grazed knuckle count ends up as