I use ticketmaster.co.uk and seetickets.com for all mine.
As said above if you want Robbie tickets you've got two choices:
1. Pay the scalps
2. Win a competition
I spent well over two hours on the morning the Robbie tickets went on sale using a landline, a mobile, a PC connected to ticketmaster.co.uk and a PC connected to seetickets.com and came away with nothing.