You are likely to lose your old address after 3 weeks or perhaps 3 months, the support finishes when you leave.
Use Thunderbird, download from the server. Thunderbird should get all your emails but perhaps not sent. This really depends on how you controlled your tiscali account.
When TalkTalk took over it all went pearshaped. My old BT account, (btinternet.com) was closed down within 3 weeks when we went TT.
TT was so appalling at our old house we went Virgin on moving, fios.
All my accounts and inbox were saved in Thunderbird
Contacts and inbox should be seen, but sent folder may not work from the server end.
Create new gmail account in TB, problem solved because you have tiscali/TT addedd to address book