Welcome.
A little more information would help us as mentioned
here but thankfully I managed to find their website
www.manheim.co.uk.
A 192.com search reveals that they mostly use 0870 except Manheim Auctions Ltd in Leeds have a geographical of 0113 282 0707.
The one in Gloucestershire has a geographical number of 01453 542 939.
I would try these if they are of any use to you but I couldn't find one in Manchester though.