If you are delayed and arrive after the gate closes Eurostar and your current train company should arrange you a seat on the next available train. 

The 45 minutes is a guide to ensure you have time to pass security etc. I don’t know how precise Eurostar is on closing the gate as they do not start boarding until about 15 mins before departure.

It happens very, very often that the ICE from Germany-which brings many more pax to change-is late-or very late and all reports tell that the staff at BRU i very well used to that and tries to accomodate it as much as they can. But there is never a 100% garantee. Delays on your train are not very frequent-and on normal weekdays it is also possible to cut off a little by changing at Berchem (just after the tunnel in Antwerpen)-mostly same platform-into a direct NMBS IC to BRU-the BeNeLux takes a detour via airport and thus takes more time. These IC are very frequent-4/6 tims/hr.

Thanks for the replies! Much appreciated, I managed to make the connecting train here in Belgium! Yeh strange the say on one app 45mins and on other sites etc says 30mins. 
 

Regards Alex

Like politics, it is all in the subtlety - Allow 45 mins to go through check in (suggestive) and gates close 30 mins before (Instructive and precise).

We took about 10 mins from entry to lounge when no queue, Others take a lot longer when queue builds up.