Got a 10 litre plastic tub for them yesterday so they should be sorted, holding heat much better now and they seem to be eating more.

Just one last question though: I can only provide 35-40c during the day, at night it dips to around 27c but the roaches are always dark so will it matter?

Have put the roach tank on top of my lighting hood and it has a heat mat. Being on top of the hood really pushes it up but when using the heat mat only; it's an average of 25c.