[mnovikov]

Just to put it in proportion, I have tried the 150/2.8 FA on 645D and a distant target and I didn't see any loss of sharpness. I would say, it's my favorite lens overall without any obvious flaw, very bright, light, focuses well, very sharp. Portraits come out just great. The minimal focusing distance is not short enough for very close shots if you want to fill the whole frame, though.
The old MF 150 is completely different - very sharp, compact and light, but dimmer and is intended for tele use.
I am not sure, what would affect the quality more - cropping or the diopter lens in the front.
Extension tubes don't support autofocus but they do support auto exposure so it is a decent option with possibly the best quality overall; can anybody really compare?
I have a new in box (old stock) lens and the tubes for sale but have no clue about customs and shipping to the UK.
KEH is very good.
My experience on ebay is overall very positive; both the lens and the tubes pop up regularly there.
Best luck! Post some pictures here after you get to use it!

[Smolk]

I had the 3.5/150mm as well, which is sharp indeed, but its minimal focus distance is worse than the FA 2.8/150mm. I cannot compare the sharpness of the 150mm lenses though, I sold the 3.5 too early (and too cheaply).

[Normo]

Pardon my ignorant question, but on the earlier reference to the 150mm lens at KEH, there was a (67) following the lens name and description. What does that refer to? Thanks Norm

[Normo]

Never mind, figured it out. Filter size! Norm

[Smolk]

But there is a 2.8/150mm lens for Pentax 67 as well, which is much older afaik, 6x7 in the name but ads may mix that up