is it safe to .......

use an extension reel to plug into an out side socket and trail down to my shed to heat 2 propagators?? I will keep the cable off the ground as I have dogs but is the cable ok exposed to the elements it is a proper reel extension lead.

Proper external extension leads are best but most modern cable should be OK unless you are leaving it out there for a really long time.

Ensure the end inside the shed is protected from water ie get it off the deck and it is recommended to wrap the block and propogator plug in a poly bag to exclude damp air.

Very important is to ensure of course that the extension reel is rated for the propogator current (unless you have some great umpteen kilowatt beastie of a propogator, it should be OK but only takes a second or two to chek and be sure)    Most extension reels recommend also that you fully unreel them although for low current appliances it should not present a problem if you don't.

thanks for your replies we have just had a brand new "fuse box" fitted and yes I should have asked the electrician then !! but I forgot  but I will put the reel in a plastic bag at the shed end as stated again thankyou