I have a big-ish downpipe collector/filter in 87mm steel downpipe. Problem is I live near a couple of conker trees. It only takes a couple of those huge leaves to block said filter. Next downpour and it's going all over the wall.

Most cheap downpipe filters have too small a throat for the deluges anyway.
Most also use a filter straight across the entire flow. - Which spells blockages to me.

I really like the look of WISY collectors, which can't block by design. Nice. But at about £200 this really is a lot to spend to fill my butts. eg here FS 87 VA for example:
[url]http://www.constructionresources.com/products/pdfs/services/WISY%20filter%20collector.pdf[/url]

I could try to make something which works the same way, but my time is precious (as is my cash).

Has anyone come up with a non-block more economical alternative with a similar parallel filter design? (Or perhaps a nice DIY design which might would work with some suitable pipes and solvent weld)

I made a leaf filter for my downpipe - the water passes staight through, leaves get washed out sideways - mostly. It does get blocked sometimes but fitted at head height it is easy to clear. The wires are either tinned copper or thin galvanized steel, poked through small holes drilled in the pipe front and back. Here comes, I hope , some pics:-

That's the sort of level of technology that even I may be able to manage!

Rgds

Damon