Here's the beginning of the poem [still under copyright]:

Dear Mum
While you were out
A cup went and broke itself,
A crack appeared in the blue vase
Your great-great grandad
Brought back ...

Start with the obvious: it's a poem pretending to be a letter. It's addressed 'Dear'. The letter's addressed to 'Mum', so we can assume that (given the form of address) it's likely that the poetic voice (or, in this instance, the writer of the letter/note is a child). 

It's a comic poem, as we learn immediately: we're told (as Mum is told)

a cup went out and broke itself

We know, as Mum will know, that it's impossible for a cup to break itself, and that our protagonist is responsible. So too is he responsible for everything he tries to absolve himself of in the poem - and that's its joke. Yet here Patten's also making a poetic joke: he's using a type of metaphor - a poetic lie - called personification to embody the protagonist's thoughts. Personification pretends that inanimate objects are alive, and this poem has several as well as the self-breaking cup: magically appearing jam stains and self-cracking vases. It's like a magic imagined world has come alive.

There's a growing irony too in the poem: the more the poetic voice claims innocence, the more we realise they are guilty. 

In terms of its form, Patten employs shifting irregular line lengths and no strict verse form or rhyme scheme - though he does end his poem with a rhyming couplet, which just finishes it off with a flourish: 

knowing you’re going to have a fit,
I’ve gone over to Gran’s for a bit.