On a .8, post-92 national contract, you shouldn't normally have more than 432 hours contact time (assuming you prepare these classes and mark related work).
You should therefore be exceeding this only as an exception and should not have significant additional responsibilities.
There are details that might matter. If, for example, you're structuring lots of your teaching around small group tutorials by choice, without official requirement, these wouldn't be normal contact hours. Observations don't always count as contact hours. Demonstrating and ... um ... life modelling etc don't count as full contact hours in this connection. If a lot of your marking is live feedback and you're calling that teaching hours, grey area.
Still, my answer is, probably almost certainly yes, you're being screwed over. You may want to ask:
Is your workload typical for your dept?
Does your contract stipulate hours?
Does your department have a workload model?
Are they filling it in right.
And ... Are you a woman? (The invisible academic admin / care trap)
.8 is a terrible fraction. Not your fault. I bet they think of you as FT and good with students.
If most of your teaching is anything - go into room, speak to or with class about something you've prepared, eventually mark their work outside class time - you need to review your contract and any workload models, talk to your union rep / Head of Dept / HR contact and ask about workload allocation.
If I can help more please let me know.