You will need to contact the owner's representative to arrange access.
In the case of this object this can be done by:
Writing to Nigel Shields at The Estate Office, Althorp, Northampton, Northamptonshire, Nn7 4hq
Emailing Nigel Shields:
Reporting A Data Error:
Reporting An Access Error to the HMRC:
Portrait of William
Maker:  Van Dyck, Sir Anthony
Sir Anthony van Dyck: Portrait of William, 2nd Baron Spencer [1590-1668], full length, in armour, holding a baton in his right hand, with his helmet on a table, 79 1/2in. by 50in.
Currently kept at: Althorp
THIS ARTWORK IS LOCATED IN:Northamptonshire
As per Althorp website. At other times by appointment through the contact point.