Your flowering cherries roots will continue to grow as long as the tree is alive. As a rule of thumb the roots will extend at least as far as the canopy does.

The other thing to consider is that cherries have a lot of surface roots around them, often extending well beyond the canopy of the tree.

Very true.

what i would do is not bury it at all . build some sort of wall or flower bed round it . have a slope of cobbles etc you need to find some polystirene thats the size of the bace of the pond and an inch or 2 thick maybe more depending on how close . ow back fill with soil when you wall it in and put a piece of old synthetic carpet under the polystyrene and take obviouse stones etc out . a liner wouldnt of matterd but being an expensive moulded stiff liner it wont give easily especialy on the corners if you sink it or even left as is just not so many .my pond is all above ground but a liner and apart from footings know digging just a couple of planters full i built with help .i mean you could use just a couple of inches of carpet if you cant find polystyrene tyles etc .when its got 800 gallons or so at 10 pounds a gallon pushing down i wouldnt tempt fate mate and you can see your fish easier . a couple of seats you can hand feed them . if you built like a bigger wall than the pond but level so you can have a flower bed,seats etc round it you could support the pond with a few columbs of brick or block the heavy ones are best to support the lip in criticle places . you wouldnt see them at the end then .i wouldnt risk roots and plumming it in near them . id get rid of the tree or move it away if i realy want to sink it partly realy . anyway i hope ive given you food for thaught . im know plantsmen but i do know about ponds lol . mines 2000 gallons with a big waterfall and planters and seats next to it . you can see it on my blogs somewear and give yourself some ideas on what yours will look like and it will be safe .

do you know how to make a spirit level for big ponds ? if not what you do is get some clear pipe a few feet longer than your pond . put it in the pond with water in it so it rests on the bottem and 2 of you can hold it upright at each end . you just make sure the water level at each end is level with the lip of your pond at each end or move the liner till they are and its done . id also part fill it so it settles in but you can still move it slightly then block it up a week later and top it up and backfill or what ever your doing .

Thats funny I thought I had replied to you all? Thanks for All your info.
Awww! Nosy, you are full of useful info, like the bit about how to make a spirit level, we are going to put the new pond where the other one is, which means not so much digging, we will buy some new railway sleepers to put round it two high, we will need 8 with some biggish rocks on the corners, Barry my hubby will be getting the sand from work for free which will keep some of the cost down.
One more question, I filled the paddling pool with tap water, pond water, and rain water and it has stood for over a week with pond plants in, will the fish be able to go straight into the paddling pool till the pond is sorted?
:0))

yes i cant see why not as long as theres not to many . check them and if there gasping put the hose in and overflow it . be easier if you were moving the pond and you could just swap over .you could do with covering you padling pool to keep them calm and stop them jumping and ofcourse heron .maybe you could just trickle cold water in constantly . id get it done sooner than later . good luck buy for now xx .