Upside-down cake

So, in order to get through the rhubarb in the garden without repeating myself too often, I spent some time today looking at recipes and decided to attempt my own variant of a rhubarb and custard cake. I started off by roasting about 800g of rhubarb, chopped in 2cm lengths and sprinkled with 150g of sugar, in a shallow tin at 180degC for 30 minutes, which reduces the water content greatly and ended up with about 600g of cooked rhubarb in deep pink syrup. I drained this, reserving the syrup. Next, for the cake, I lined a 23cm springform tin with baking parchment, made some custard (11g custard powder, 13g sugar, 160ml milk) then creamed 250g margarine with 250g sugar and 5ml vanilla extract and added 4 eggs, one at a time, before adding 250g SR flour mixed with 1/2 teaspoon baking powder and about half the custard, plus some of the syrup from the rhubarb. Half of the mixture went into the tin, and after spreading it out I topped it with half of the rhubarb, then put the rest of the cake mixture on top of that, in rough spoonfuls, then the rest of the rhubarb on top of that. Then I spooned blobs of the custard on the top, and it was ready for the oven. Then, disaster. While I was lifting it into the oven, the base of the springform tin sprung out. The cake mix ended up all over the oven door. I grabbed 3 2lb loaf tins and liners (thank you, Lakeland) and hastily scraped the mix into them, so all my careful layering went completely to pot. After washing the oven door, I put the tins in with crossed fingers and baked them for about 50 mins at 180degC. We had one for dinner, served with the leftover rhubarb syrup, and although it did not look pretty it tasted good.