Not very likely that it is Spatterdock. Spatterdock can be found in the wild in the UK. More likely Nuphar Japonica which is commonly named Spatterdock although it is an entirely different plant.
I hope you have a big tank. lol. This plant grows huge!!!! Within 2 months with ferts and even in low light it will span 3ft diameter!!!!
Also beware that you should decide before planting wether you really want this plant because once it has become established it will grow roots 2ft+ in all directions meaning to remove it you will have to either remove the substrate and all other content or risk leaving the roots in (which will regrow. lol)
The tuber should look similar to the centre of a cabbage/cauliflower. Imagine the plant you have as being a section of this cabbage.
With the tuber you probably got it may have gotten quite mushy around the edges. Even if it hasn't get a sharp knife and trim it so that the outer of the section to be planted is nice and fresh. To get it to anchor easier you can cut it into an egg timer shape:
-----
\ /
/ \
-----
Push the whole tuber into the ground leaving the stems to come from the substrate level.
This is the freshly planted example in the centre:
This is after 2 months and was having 3 or 4 leaves chopped away at the crown every fortnight. It was in a 125Ltr tall tank. Hard work and heavy pruning plus notice that the large leaves shade the tank a lot.
This plant eventually came out of the tank after 4 months to make room for other species to grow as it is so greedy for space.
Good Luck
Andy