The group embarked on their Progress tour across the UK and Europe last week marking their first tour since Robbie Williams' return to the band following his acrimonious departure in 1995.
And Mark, who has remained good friends with Robbie throughout the years, said that seeing the Angels singer up there with them really is an achievement.
He told The Sun, 'It was good seeing Rob doing the old dance routines. It was a sweet moment for us.
'He's been really good, you know. He's worked incredibly hard on the whole show. He has been incredibly enthusiastic.
'There are a lot of those moments when you have a look up and there are five of us. It sounds a bit eggy, but you do go, 'Wow, look at us'.
'We've come all this way and we are all here now after all this time. It's great - a really proud moment for us all getting up there again together.'
He continued, 'We forget, because we've been hanging around together a lot, the fact that all five of us are up there is actually a great thing - a really brilliant thing.
'I'm made up. To bring us together with Rob has been a challenge.'