I have removed the two Cardiff numbers, 029 2042 8100 and 029 2042 8635, referred to in the OP as they are not recognised. I know, it's only taken two years to do this!
A call to 0845 915 1515 says that callers can get more information from the website at
www.thepensionservice.gov.uk/winterfuel. This is the old URL and helpfully only redirects to The Pension Services' homepage.
The new URL is
www.direct.gov.uk/winterfuelFrom there, you can e-mail the helpline using the online form from
the link on this page.
The main Winter Fuel page has a form to fill in, and hidden in the guide to filling it in is an address to send it to:
Winter Fuel
Unit 16
Coalfield Way
Ashby de la Zouch
LE65 1JF
A search of our forum for LE65 returns just one thread which also relates to Unit 16 Coalfield Way. That thread is for De Agostini UK and it is
here. It turns out that this is an outsourced call centre provider
Teleperformance (MM Group).
A search of the SAYNOTO0870.COM forum leads to threads for
Environment Agency's Floodline and
Identity & Passport Service. The latter says that they called from 0117 915 1900 which identifies itself as Teleperformance.
Teleperformance's public sector page makes mention of the work it does:
http://www.teleperformance.co.uk/publicsector.phpThe International Pension Centre, whose
contact details are here give a Newcastle-upon-Tyne address and 0191 contact telephone numbers.