So, I thought I should revive this old thread to thank you all for your advice!
I went for Lowell's setup: I found a second hand K300F4 at a very affordable price: It's old, it's ugly, but it works wonderfully!
I bought it Saturday, a misty grey Denmark-at-its-January-worst sort of day, but today the sun was almost coming through for a moment, so I went and found a bird to test it on.
Here are two sample shots: The first is with the 1.7TC, so at 510mm, and the second is just the "pure" K300F4 after I managed to creep a little closer to the bird. The difference is recognisable, but it is definitely a useful setup, and the best (and only) 510mm setup I have ever tried!
Now I just need to get used to the all manual exposure settings, but I have a feeling you would tell me I should welcome this chance to learn my craft properly, Lowell?
Again, thank you for all the advice! I might go for the "modern" weatherproof DA300F4 one day when I can afford it, but this was 10% of the cost, within my budget and lots of fun! (and learning too...)