Which HMRC office deals with SEIS?

SEIS and EIS are dealt with by HMRC's Small Company Enterprise Centre ("SCEC").

Their contact details are as follows:

Small Company Enterprise Centre Admin Team 
SO777 
PO Box 3900 
Glasgow 
G70 6AA 

Telephone: 0300 123 1083

Email: enterprise.centre@hmrc.gsi.gov.uk

The best way to contact this branch is by email.