A picture.....
I removed one of the exhaust valves and took out the clip, collets, spring cap, inner and outer spring, rubber valve seal. valve, bottom retainer and this bottom "washer" marked "this side up VSA 106". I still cannot tell if a hardened seat has been inserted, so will replace the valve and clean up the combustion chambers for a closer look.
Steve

I've burnt more exhaust valves with unleaded seats installed (two ) compared to no seats (nil) but have suffered massive valve seat recession without them.  Lead memory often contracts Alzheimers...

As soon as you start having to adjust the tappets because the gap is closing up, you can be sure that the valve is headed northwards through the head, and it's not long before it's scrap.
For the sake of a few pence per fill, using an accredited additive is a no-brainer.

Mur - you sound a lot like me. My first instinct was to find a better head but it seems that the 12G206 from the MG1100 is the same as used in the early 998 coopers with the bigger valves raising the suspicion that there isn't much to gain - at least the way I will drive it. Of all of my classic cars, this is the one which gives me most angst but only because the series 1 body, interior, glass etc. are in such great original condition. As the engine is not original (thank God) I could go the route the previous owner seemed to be going which is to make this a sleeper and upgrade the snot out of the mechanicals. My latest plan is to make this a Monte Carlo tribute car. It is great to have the resource of this group of people!
Steve

I have put many tens of thousands of miles on 998s burning unleaded fuel with unmodified heads at modern highway speeds and only had a few exhaust valve failures at about the same frequency as they failed when fuel had lead in it. 

If you re-grind the seats and the valves then the hardening already present from years of use will be gone and you will probably have accelerated wear, which might be held off by using an additive. 

Obviously, spending money on the car needs to make sense. Not having the support of a good local community and machine shops that are keen to work for those markets is certainly part of considering these economics.

The present day version of me seems to spend money on cylinder heads being rebuilt by true craftsmen. The younger version of me would change the stem seals and drive to California.

Agreed. Unless you plan on cruising the interstate for hours at highway speeds leave it alone.

I'd leave it exactly as it is, and use a quality lead additive instead - Castrol Valvemaster, RedLine and Millers are the only three products that passed the stringent tests for approval over here.

The cost of getting the seats installed and the risk of the head cracking compared to your usage are greater than the cost of using an additive.

Thanks Spitz. With the rocker shaft off, I turned the head over and filled each combustion chamber with alcohol. No leaks at all. I can also see that the slightly dished pistons are +020" so the engine clearly had some work done before I bought her. Combustion looks to be very clean as it took almost no effort to clean the piston tops. Mainly carbon from running her on choke for a few minutes each time I moved her around the garage.
If I remove the exhaust valves, is there a way to check if the hardened seats have already been put in there?
I live in an area short on Brit car experienced machine shops so am currently leaning towards using additives rather than risking operation to insert the hardened seats. The car will not be raced but will be driven in a "spirited" fashion and is likely to do less than 1000 miles/year.
Steve

They would not have been unleaded heads ( ie: hardened valve seats )
If you've got the head off and doing work...have them installed IMO

I took the head off my MG1100 engine yesterday (running in a series 1 Austin 850 body). It looks to be in great condition, despite burning a bit of oil. I was planning to have new exhaust valve seats installed but read an article on the forum that seemed to indicate that the 12G206 head - also fitted to early 998 Cooper S engines does not require modification. I cant be sure because the article referred to engine numbers rather than head castings. Can anybody out there confirm or deny this?
I do plan to pull the engine to check bearings and ring/bore clearances but if the bores look good, I will just run a hone through and put it back together after grinding valves in.
Thanks in advance
Steve