leaking mains valve in hot press - how do i fix?

The gate-valve for the cold water supply, piped from the cistern in the loft down through the hot press, is leaking. The valve is in the hot press. Copper pipes.

Would anyone know why this is, and how i can fix it?

If i get a plumber, how much should it cost to fix and how long should it take to fix?

All suggestions greatly appreciated!

The gland packing in the valve is leaking; you could try and loosen the nut that retains this and put some ptfe tape in there; twist the tape to make a "string" which is then wrapped around the spindle, replace the nut and see if this works. Alternative the fibre/ plastic washer which is used at the join of the vlave body may be at fault. It's a very quick and easy job to replace the valve, a new gate valve would cost about €6 an even better lever (ball) valve would cost maybe €10, labour would be extra of course. The attic tank would have to be drained down first.

thanks Carpenter!

is this a diy job or should i get a plumber? recent success with syphon and cistern going to my head!

Depends, how handy are you? If you want to replace the valve buy another gate valve (should fit without altering the existing cap nuts, olives etc) and remake the joint using a little Boss White. This is a very straightforward job, I'd say go for it- if you're competent!