Retentions-invoicing & VAT

query regarding invoicing where there is a retention to be made

Situation is :    regular billing of £400/mth fee, subject to  10% retention

so

1] is it invoiced showing  (400-10% retention)=360 + vat  with retention being shown on face of invoice,  and when retention is contractually due, it is invoiced  + vat,  with  sales per period now  @ 400 [ &  retention invoice being  DR DEbtors  CR BSheet retentions with CR vat]

2] or 360 + vat invoiceable through time,  and when the  retention is invoiced much later - it is then recorded as  revenue [ there is always a chance retention may not be recovered ! ]

3] other invoicing / p+l recording

many thanks