Warranty Invoices from dealers

[Khantouch]

I had some work completed on a dealer credit offered by BMW UK due to an extensive delay waiting on a part. The work included an oil change and rear brake discs/pads replacement. On completion I asked the dealer for a copy of the Invoice and was informed as the Invoice hadnt been processed I couldn't get a copy. I've contacted the dealer again requesting a copy of the Invoice and have now been told as all work was completed under goodwill with no charge to myself I don't have the right for a copy of the Invoice??
Is this true, as it's my vehicle and surely I'm entitled to Invoices detailing work on my vehicle to make up its service history. I suspect the work didn't make up to the dealer credit hence their reluctance to give me a copy. Any advise or tips on this matter would be appreciated