Trader Joe's sells frozen mini-croissants. The instructions for them are, as far as I remember: set them out overnight (or equivalent) on a lightly greased cookie sheet to "proof" (or "rise"). In the morning you may want to brush a thin coating of egg wash on them before cooking to make them shiny & beautiful (this is optional). Then cook in preheated - 350 degree F - oven for 15 to 20 minutes or until croissants are quite golden brown. (TJ's says that if they are just light brown they are not done on the inside.) Then they should cool for 10 minutes before serving.
The "mini-croissants" are actually fairly big in my opinion. Not sure exactly the size of the items you have, but if they are significantly bigger than those "mini"s, you might need to add a teeny bit of time, but 15 - 20 minutes is pretty vague anyway, so I would say just watch the color. Dark chocolate-brown is a little too done; and pale beige is not done enough.
Alternatively, have you checked the website listed on the plastic bag they came in? Maybe there are instructions there. I can't really read it in your photo, but it starts out "www...."!
OK, new UK info; in case these are "Field Fare" all-butter croissants,
Spread out on a baking tray and leave to prove overnight at room temperature. Egg wash and bake in a pre-heated oven at 190°C/375ºF/Gas Mark 5 for 8-10 minutes, until golden brown.
These cooking instructions are a guide only. Your appliance may have a different power rating, so please adjust accordingly"