Typical train times (based upon travelling on Wednesday 10th June, which I chose as a random example):
Depart Penzance 0645
Arrive Paddington 1224
Take an eastbound Underground train on the Circle or Hammersmith & City Line, for 5 stops, to Kings Cross/St Pancras, arriving at international check-ins at around 1250 to 1300.
Take the 1404 Eurostar service to Brussels Midi, arriving at 1705.
Go to Platform 4 and get the 1728 Thalys service to Köln (Cologne), arriving at 1915.
Then get the 1926 Inter-City service to Hannover, arriving at 2228.
All sourced from here, which brings together all European rail timetables:
http://www.bahn.com/i/view/GBR/en/index.shtml
To check prices, and to book, call SNCF UK on 0844 848 5848 (Weekdays 9am-7pm, Saturday 9am-6pm).