Place the shells and heads in a pot of boiling water ~1.5 - 2 litres and boil for about 25 min to make a broth (add fish stock of you have it)
When done strain the stock and discard all the bits.
Meanwhile dice the garlic and the onion, grate the tomatoes.
Chop the pepper into small 1 cm chunks
Heat in a large pan (sorry I'm Scottish I don't have a paella pan) 2-3 tbsp olive oil
Brown the prawns on either side and save for later
In the same pan fry the onions on a medium/high heat till soft.
Add the garlic and once fragrant add the peppers and cook for a few minutes, stirring regularly.
Season liberally with salt and pepper
Add the grated tomatoes, the saffron and smoked paprika.
Stir thoroughly and cook out the moisture
Add the rice and ensure to spread it evenly. Once spread pour over the broth till 2x the amount is covered.
Don't touch the rice and cook on a high heat for 5 minutes
Turn down the heat to medium and cook and add the prawns on top.
Continue to cook until the rice is cooked and the water has mostly evaporated, 10-15 min. (Remove the prawns if they start to over cook and place them on to serve)
The rice should be soft and there should be a "socarrat" or a slightly crispy/savoury base on the rice.
Notes & thoughts
I eyeballed this in a big stainless steel pan, hope it works again the next time.