On 07/08/2014 22:56, Nightjar "cpb"@ insert my surname here wrote:

I don't like plastic rainwater goods. I much prefer cast aluminium, such
as Alumasc:

http://www.alumascrainwater.co.uk/

You could probably do worse than contact their technical support, say
you want to match an existing CI downpipe and ask them which of their
products is the right one.

I'd be cautious about aluminium in contact with iron. Make sure you have
a gap of some sort - a good thick coat of paint might do - to prevent
any funny galvanic stuff going on.

Andy