Are you SURE you have a Mk1 and not a MK1*? If you have a Mk1*, as I suspect you do, then there will be no bolt catch but a simplified bolt release with a cut out in the receiver.
Here's what you do ...
1 - Turn the bolt upwards and left as if to open the bolt.
2 - Withdraw the bolt slightly until you see the bolt head align with the cutout on the right side of the receiver. You will have to push the bolt forward with your thumb to align it and you'll feel some resistance.
3 - When you have aligned these perfectly flip the bolt head upwards to the left and then withdraw the bolt.
4 - If you have a Singer Micrometer Sight and not the battle flip sight you will need to lift the ladder sight up before the bolt can be removed, otherwise it will not withdraw.
5 - When replacing the bolt you need to push the bolt forward with your palm until you align the bolt head with the cut out, then flip it down in place and you are good to go again.
