I've only took a few shifts while theyre short staffed. they have hired my replacement. I had left for 2 weeks and then they called me with shifts. How do I use holidays if I only get shifts on short notice? I don't understand.

I gave 4 weeks notice over 2 months ago now and should of received 4 weeks pay plus holiday entitlement. I did not recieve to holiday pay but they said they would investigate it. It dragged on all month and I did a few shifts for them after my notice was served to help with staff shortages. Today I was paid for the extra shifts I did for them but still no holiday pay. If they are saying I am still staff (bank staff) because I'm helping with a couple of shifts can they deny my holiday pay? Even if I was bank staff I don't see how I would be able to use holiday time because you only get offered shifts short notice when you are bank staff. This firm came in in January as a takeover. I have still received not contract from them. Thanks guys. George