You have a T5 visa and you want to come early as a visitor.
This situation is covered in Paragraph 30C of the Immigration Rules, which says...
30C. An Immigration Officer may cancel an entry clearance which is
capable of having effect as leave to enter if the holder arrives in
the United Kingdom before the day on which the entry clearance becomes
effective or if the holder seeks to enter the United Kingdom for a
purpose other than the purpose specified in the entry clearance.
Generally they don't like it because this type of thing is seen as an attempt to get more leave than you were issued. Also you wrote...
I've previously been issued a visa for 6th months leave to remain when
I visited back in May, which would technically still be valid, since
it hasn't run up yet.
No, not if you were given leave to enter for 6 months or less. This is where Paragraphs 20A and 20B kick in...
20A. Leave to enter or remain in the United Kingdom will usually lapse
on the holder going to a country or territory outside the common
travel area. However, under article 13 of the Immigration (Leave to
Enter and Remain) Order 2000 such leave will not lapse where it was
given for a period exceeding six months or where it was conferred by
means of an entry clearance (other than a visit visa).
20B. Those who
seek leave to enter the United Kingdom within the period of their
earlier leave and for the same purpose as that for which that leave
was granted, unless it (i) was for a period of six months or less; or
(ii) was extended by statutory instrument or by section 3C of the
Immigration Act 1971 (inserted by section 3 of the Immigration and
Asylum Act 1999); do not need a visa to enter the UK.
They do have some discretionary latitude however.