transplant advice

I would like to try to transplant a Spirea Bridal Wreath which has grown in wrong place (not its fault - I muddled all the labels when I was planting new shrubs about 2 yrs ago)
To be honest the plant probably wont survive as I'm not green fingered, but I have successfully transplanted x2Pieris from Aberdeen to Doncaster and they are still both doing well so I'll give it my best shot.
Can anyone advise me on the best time to transplant it-
I 'd hoped to do it September but would I be better waiting til March/April?
Also any advice on the actual transplanting welcome.
Should I trim it first? I'm not sure how big the rootball is, but assume I dig down til I find the roots, then lift plant out, but do I then leave most of soil and transplant to new site with a bit of multipurpose compost in hole or should I try to get rid of soil and transplant in fresh compost?
And should I water the plant before I dig it out as well as once its in new place, and do I plant to about the same depth?

Hope these questions don't sound stupid

 

Sorry I'm not very good at this
The black post (near back) is 75cm ht
The main plant is about 100cm across ( plus a few bigger straggly branches)

 

Thanks I'll wait til autumn then give it a go

 

If I may add to JWK`s advice. Prepare the receiving hole first by forking in some bonemeal to the bottom of the hole. Also mix some in with the backfill soil. Finally, once the plant is in situ, give it a good watering to make sure the soil is in contact with the roots.

 

That is a good idea. Much less likely for any mishaps and you won't need to watch re watering if you wait until afterthe plant has dropped it's leaves. If you pruned it now you will loose all it's pretty flowers for next Spring. You may already know but it is a good idea to prune Bridal Wreath immediately after it has flowered, pruning off all the flowering stems. It neatens the bush and stops bare gaps appearing on the stems. I have 3 ancient (about 40 years old) bushes and they definitely benefit from a good haircut around May time.

 

Thanks all advice is welcome.
I haven't pruned it this year but if it survives and flowers I will next year

 

Thanks I'll get some in so I've got it while I remember