ManoMano complaint: Refusal of full refund including return cost

I ordered the item on the 8th august 2022 and it got delivered on the 10th august 2022
We was on holiday at the time.
Upon our return on the 15th august 2022 I checked the item and it had several items missing . I contacted Mano mano
The communication is very poor they basically said if they knock me £40 off I can purchase the missing items myself . The correspondence went on through a third party yet I purchased it from the company . This has been going back and forth to this very day . They are basically telling me I have to pay to return the item and that they can do this because they offered me 40 off . If I pay to return (large item ) then I will get a refund of only the purchase price . This cannot be correct surley as if the items were all there I wouldn’t have to have it returned . I’m wanting a full refund and for them to collect the item at no cost to me .