I have an old cypress pine subfloor which, while structurally sound, looks a bit dodgy (ie poorly cut ends, boards not tightly together etc) and also has over 3 mm depressions between joists and other imperfections.

How can I fix this? I understand the concept of floor leveller but will it work on boards? Won't it run through the gaps? Can I staple something (ie cheap plastic) on the boards and lay the leveller on that? Will it fragment under the floor over time?

Also, is it necessary to glue down click lock engineered floor? Any point? The floor will have skirting boards added after it is laid.

If I had the cash I would pay a professional but the renovation mistress, she does love the high life and I have other bills to pay.

Any advice would be appreciated as I hate to do a bad job when a simple question could prevent it.