Hi,
This might be a long shot for you but look on ebay for a guy listing as Beechcroft diving. I've got stuff from him before and I'm pretty sure he does pumps a lot cheaper than that. And they are 'tweakable' more easily.
He also does poles and stuff if you are interested. I only really know because I do a bit of diving with him, or used to, and he imports diving kit from Taiwan/Vietnam/etc. Think that's where he got the trolley systems he sells.
John Hewitt is his name, his brother used to race side cars with Steve Webster (not that that has anything to do with window cleaning)