Thanks Ian,
apparently it just needs a drop of lighter fluid to clean the mechanism and then a couple of very very small drops of sewing machine oil - taking the top off the camera looks straightforward but of course has to be done care.
The last time I had a CLA I am sure it was around £100, certainly less than £150, but that was over 20 years ago and it could have been longer - it is probably 4 or 5 times that now
The problem is that the slow speeds 5, 2, 1 are inconsistent, (but I cannot remember ever using ½. 1 or 2 seconds), - there are a few utube vids on the subject so it must not be unusual and of course the M3 is years old so it can be expected
From what you say, it sounds like the lighter fluid is to soften old gummed up lubricant, and the oil is to try and make it move freely again.
Doesn't sound like a complete or long lasting repair though.
I have never worked on a Leica, but I believe it is pretty similar to the Zorki etc.
The shutter speed accuracy is dependent mainly on three things. The free movement of the curtains, the tension on the curtain springs, and the free movement and accuracy of the release cams/pawls on the speed control assembly.
If the curtains do not move freely, it is usually the high speeds that are affected, ie 1/1000 will be nearer to 1/500, which probably would not even be noticed by most.
If the curtains are not free to the extent they affect the slow speeds, then the high speeds will certainly be out too.
Some people increase the curtain spring tensions a bit, which may help, but then often makes the speeds below 125 to fast.
The lighter fluid and oil repair would probably apply to the delay control under the top cover, in which case if done carefully, and no lighter fluid or oil gets anywhere except where it is supposed to, it shouldn't do any harm, but as the gummed up lubricant will still be in place, just softened, I can't see it being a 100% repair. If it works OK for another few years, then I guess it would be OK.
I think if you are going to try it, more research on peoples opinions of it and how effective it is would be worthwhile, otherwise it would be best to pay to have it done properly.
On the Zorki/Zenit it takes 2-3 hours to remove the curtain assembly, clean completely, lubricate and fit new curtains or curtain tapes and do the speed control, so that may give an indication of the cost.
There is a big difference in the current and future values of the Leica compared to a £20 Zorki though
I do this with every film camera I buy, I use slide film as it has less tolerance to exposure errors and is easy to "read". It is expensive film but I feel it is worth the expense for the peace of mind.