Now the tailgate won't open!
Checked the fuses, done a factory reset and still can't get into the tailgate. 
Isn't there a manual way of getting in ..... like a facelift Omega saloon
No. 
You could try this as its just a new battery on it....
disconnect the battery and see if it resets
If that doesn't do it....
if you need to get into the boot urgent you need to drop the back seat and in the middle of the boot lid trim on the inside is a small grommet, pop that out and inside is a slot for a flat screwdriver, turn it anti clockwise and up pops your boot lid
Then
Technical Service Bulletin 2820
Subject: Tailgate / trunklid, lock - Does not open
Brand: Model: Model Year: VIN: Engine: Transmission: from | to from | to Opel, Vauxhall Insignia 2009 2011 91000362 B1134852 All All Opel, Vauxhall Meriva-B 2010 2011 A4000001 B4242704 All All
Option Complaint Tailgate or trunklid does not open. Cause Defect of the tailgate/trunklid release switch. Production
Remedy
In case of customer complaint that tailgate or trunklid does not open,
do not replace the latch assembly, striker, control module or wiring
harness. Do not attempt to re-align the latch assembly or striker.
If DTC(s) are not set, replace the tailgate/trunklid release switch.
For details see Technical Service Bulletin 2778 / 2779.
FunctionalGroup C - Equipment/Windows,N - Electrical Equipment Complaint Group 13 - No/Poor Function Trouble Code None
I guess it may have failed because of water getting into it (has its been raining a lot recently?) seems a common problem....