Bosch built in oven struggles to maintain temp?

Refers to HBN412, main oven. Thermostat and fan appear to working well. Door is a little loose in closed position. Seal is intact but probably needs replacing. Appliance voltage is OK.

My guess is that the door closure is the issue. How can I tighten the door e.g. are the closure springs replaceable? Will a new gasket fix this? There doesn't seem to be any means apart from the hinge springs to hold the door tight closed -- we can get the oven to work better if we jam it with a length or timber.