Thank you all for the replies.
After a bit of thinking about it, I think I've come up with a solution: Lime now, plant as usual, and feed with teas. The only problem is I still have not come across a source for lime. I'll have another look tonight, but if someone could recommend a source it would be very helpful.
agapanthus - your soil sounds like a gooey nightmare! The one thing you you can console yourself with is that clay is packed full of minerals, so if you can tame it, you should get excellent growth!
What I have been doing where I have found a lot of clay, is removing most of it (and putting it aside till I can figure out what to do with it), and leaving a few clods aside to dry out in the sun. Once dry, it crumbles easily into a fine powder which I sprinkle over my beds.
Luckily most places where I've sunk a shovel don't have too much clay, just a 6-12" layer about 1-2' below the top soil. Had a surprise today though: below the clay is a thick layer of what looks like fine gray gravel, which I assume is an alluvial deposit of ground up slate (which seems to be the bedrock round these parts) from the last time a glacier was passing through. I'm guessing that should be packed full of minerals/nutrients too, so I intend to bring some of it up to the surface soil layer.
shokkyy - nice to hear someone is using similar amendments to those I am using, and that they helped. I have the non-calcified stuff too, and I'm wishing I had some more, but it's quite expensive stuff. In hindsight, I should have passed up on the rock-dust (I didn't realize there would be so much ground up rock round here!), and bought another big sack of seaweed.