Firstly, I'll get the ball rolling with some basic details.

Website: www.giantgroup.com

Contact details including address are here.
1 New Oxford Street
London
WC1A 1GG

A look back at Giant Group's website on Internet Archive reveals a freephone number, still in operation:

08000 92 10 11

. If you wish to call from your mobile, it may be best to avoid this route as most (all?) providers charge for these numbers. Sometimes they are out of inclusive minutes, sometimes they cost more than a geographical number; you will have to check your tariff.

This page gives another (maybe old) address and contact number:
Angel House
338-346 Goswell Road
London
EC1V 7QN

T  020 7 520 0500
F  020 7 520 0555

A search of the Giant website for the London dialling code, 020, reveals this number on a few pages (results here). Unfortunately, both these numbers have the standard network announcement giving the 0870 numbers.

See http://www.giantgroup.com/]this page[/url]. It is from March last year and says that they have moved. At the same time they have adopted the 0870 number. 

A look-up of the registration information for giantgroup.com reveals nothing as it still has the old address with a DDI telephone number 020 7520 0597. 

Few companies here...

Giant Group Plc
Giant Powerhouse Ltd
Giant Consulting Ltd
Giant Powerhouse Contracting Ltd
Giant Accounting Ltd
Giant Services Ltd
Giant Directors Ltd

1 NEW OXFORD STREET, LONDON, WC1A 1GG

Anyone got a 020 number?

Trying http://www.bt.com/directory-enquiries I get the following:

Giant Ltd
Tel: (020) 7404 8220

but that is a fax machine.

Every employees' direct dial number is given out as 0870 125 86xx

These guys are on the web as http://www.giantgroup.com

They are a company who offer temps the option to save income tax, legitimately.

(if anyone is a temp or contractor and wants a referral, please let me know )

Your directory already has an 0800 number for them, but that is not much use when calling from my mobile.

However, after I entered into business with them, I discovered it was all 0870, and my contact refused to give out an 020 number (she actually said that they didn't have one).

Cheers