From Wembley Central on that day, the Overground/Lioness line won't be running at all, and the Bakerloo line will only be running replacement buses. This means the only actual train services will be the hourly Southern service to/from East Croydon and Clapham Junction.
there is a 3 hour gap on southern journeys from wembley to clapham is this normal ?
It's likely due to the event at Wembley resulting in no trains running in that 3-hour gap. I'm not sure why, potentially because the station would get too overcrowded if they ran just the 1 train each hour from there.
And do I have to use the same company for the return (its an open return ticket)
None of the open returns from Basingstoke to Wembley Central seem to restrict you to any operator.
and could anyone please suggest any way of getting back to basingstoke as not having any luck with trainline
The 'not via London' tickets don't have validity on the tube so it wouldn't be valid via Wembley Park and Waterloo. If this option seems much easier than the below routing, you could consider excessing the ticket to a Travelcard.
On a 'not via London' though, this is likely to be the best route:
Wembley Central to Willesden Junction (Bakerloo line replacement bus), Willesden Junction to Clapham Junction (Overground/Mildmay line), then Clapham Junction to Basingstoke on South Western Railway.