Section 18 ('GBH with intent') applies here.
Read the definition of 'GBH' (or 'wounding' which is just an alternative wording) under "Unlawful wounding/inflicting grievous bodily harm, contrary to section 20 of the Offences against the Person Act 1861" here:
http://www.cps.gov.uk...s_against_the_person/
Then scroll down to "Wounding/causing grievous bodily harm with intent, contrary to section 18 Offences Against the Person Act 1861" to see what elevates a Section 20 offence to Section 18.
I'll now use my psychic powers to predict that your next question will be: "What's the likely sentence?". See pages 3 to 6 here:
http://sentencingcoun...ine_-_Crown_Court.pdf
(Note that the sentences shown in the table on page 5 refer to a first-time offender who is convicted after a trial. An early guilty plea can see sentencing cut by one third but previous convictions can push sentencing higher).
Chris