Part Cancelled Order

I have a problem which Dell are refusing to resolve.

I placed an order on 27 April for 2 PC's, 4 monitors and some peripherals. The 4 monitors were delivered on 29 April but I received an email to say that the rest of my order, (which they'd split into 2 separate orders) was delayed and would be with me in a few days. Then a few days later I received another email telling me that my order was further delayed and would be with me the following week. Then a week later, you guessed it, another email saying the order was now delayed even further. Eventually on 16 May I received an email from Dell saying that they had cancelled my order. So now I have 4 monitors and no computers to use them with. I reordered 2 PC's from Dell, and 2 new 4K monitors as one of the PC's was a higher spec than the original ordered one and could use 4K monitors. However, Dell are refusing my return request for 2 of the original monitors as it is outside the 14 day return period, despite the fact that they didn't cancel my original order until the 14 day return period had already passed.

This seems grossly unfair as the only reason I need to return the monitors is because they cancelled my order.

Any advice as to how I can get this issue resolved would be greatly appreciated.