do you mind if I ask how you came to the conclusion of the fault?

So far by just describing the symptoms to the UK MX5 mailing list. The techies guys there are all pretty much in agreement that it is almost certainly the thrust bearing.

is the car 5 speed or auto?

Manual, from what I have read it probably wouldn't have happened with an auto, because it doesn't put the same backwards and forwards pressure on the crank shaft.

what are the symptoms?

Started making a diesel like rattle between 3000-4000 revs a few months ago. The other day, 10 miles from home, it the noise got louder. It got progressively louder over the 10 miles.

By the time I got home I sounded like some hitting an iron radiator with a large wrench repeatedly. The sound is present with the clutch in or out and various with engine speed.

Laying under the car the sound (and associated vibration) appears to come from the lower part of the engine block.

just how much was the estimate?

One quote so far of £900, without tax. Plus the engine at £250-£350, plus the cost of transporting the car 40 miles across London.

Because it is a relatively young car but has done loads of miles (most MX5s over here manage about 5,000 a year, I have done 20,000 a year for seven years) it is worth bugger all. If I was going to sell it I would need to replace the roof and probably the rear fender as well.

I don't mean to question your knowledge of such things, just that I see a lot of good cars tossed out because the owner was misguided.
thrust bearing failure is not all that common on a motor that age. and rare overall. not unheard of, just awful rare.

That's alright, I'm planning to get someone to have a look at it before I do anything drastic with it (like scrap it).

My car is a very late Mk1 1.8, which apparently means that it has a good chance of suffering from the crank shaft thrust bearing machining problem that lots of the early Mk2 1.8 cars also had.

as a side note: 30k lbs is a lot of pie. unless I'm out-of-date, you're talking ~50 grand usd?. couldn't you get any thing you please for that much?

You need to remember that cars are much cheaper in the US than they are over here. A new Porsche Boxster for example here costs £32k, which is $64k. According to the US website it is $42k over there !

That said, yes 30k will buy me a recent second hand example of most of the cars I might want to buy. It rules out thinks like the 911 or any of the Ferraris however...