[pharnorth]

I have a 7 acre hay field which was ploughed and reseeded 3 years ago. It is used just for the one hay crop a year then a very light use with my dozen sheep through to January, seems to be holding up well
I have a 5 acres field which has been permanent pasture since the year dot.  We over winter 5 horses then it grows a hay crop and the horses return after the sheep have taken the top off so has horses/ livestock on for 9 months a year. It is very compacted and was under seeded 3 years ago but yield dropped dramatically this year.
Then there are 3 mini paddocks I use for the horses while the hay is growing and through to October to rest the 5 acre field as much as possible.  Like the 5 acre,field, they were under seeded but are very compacted. 

my neighbours paddock is now empty- is it worth trying to rent it and getting my compacted paddocks ploughed and reseeded and if so when and how long before reuse?  Guidance appreciated!

[Buttermilk]

I would get the field sub soiled to break up the panning and let air into the soil. Ploughing will give the weed seeds an excuse to have a party and make the top loose so the horses will do a lot of damage with their feet for a couple of years. An old permanent pasture should be species rich and not rocket fuel grass designed for dairy cows.

[Marches Farmer]

Ploughing means the weed seeds that have been dormant for years will germinate.  If the area is species rich it may take several years for the deep-rooted species like sheep's sorrel to re-establish.  I'd be inclined to tackle the compaction.  If it's due to horses walking the fenceline or something similar putting them somewhere else and leaving the field to the worms and frost for a while may halp.

[Carse Goodlifers]

Lots of options.
First thing though - sowing grass/reseeding/overseeding - wait till late spring or the autumn before you do it.
If you sow spring - wait till end of summer before grazing but only lightly graze - it will help with tillering.  Autumn sowing, you may be lucky and lightly graze it about now.  In both case's don't allow heavy stocking until the second year.

If you can subsoil then good as it will break up any compaction issues - but this is best done when the ground is starting to dry and crack naturally so ideally in the summer.  If very wet you can do more damage subsoiling than you think.  Also watch out for any shallow drains crossing the paddock.
You could then cultivated the field surface with a grubber or power harrow, sow seed and roll.

If you plough now, hopefully the frost and weather will break down the furrows for spring.  You could them harrow the area and make a stale seed bed.  Wait for weeds to grow, spray off, sow and roll.  If you don't want to spray off cultivate again and sow on a dry windy or sunny day so that any weeds will wilt.

If you want a clean seedbed in which to sow the seed - plough down or spray off the existing areas.

It may be a case that you get in a sward lifter which will help with surface compaction, aerate the paddocks with spikes, graze short down and then overseed with a high rate of seed and then roll the paddocks.

Bit muddled order - sorry.

[pharnorth]

Very helpful replies, thank you.  I'll look into whether I can rent my neighbours field for  a while and that may determine how long I can free the paddocks up for.  I don't think leaving it to the worms will work as it gets very dry here in summer and would take a long long time to get back in order and it isn't species rich so the sub soiling seems the way forward.