Here is an explanation of the law on Sexual Assault.
When a sleeping woman wakes in bed up to find a man holding her, groping her body and trying to get on top of her then that is classified as a Sexual Assault under the S.3 Sexual Offences Act 2003 ;
A person (A) commits an offence if—
he intentionally touches another person (B),
the touching is sexual,
B does not consent to the touching, and
A does not reasonably believe that B consents.
Whether a belief is reasonable is to be determined having regard to all the circumstances, including any steps A has taken to ascertain whether B consents.
Sections 75 and 76 apply to an offence under this section.
A person guilty of an offence under this section is liable—
on summary conviction, to imprisonment for a term not exceeding 6 months or a fine not exceeding the statutory maximum or both;
on conviction on indictment, to imprisonment for a term not exceeding 10 years.