Partial ISA transfers - is this correct?

I was aware that you could transfer part of a cash ISA into another existing cash or  new cash ISA with a different provider...

Now I thought this only every related to 'whole years' - so that your provider maintained an accounting 'separation' between each tax year's subs [and the interest thereon] and so you could only ever move the money as if each year was effectively in a different account.

But apparently it's fiendishly simple... if you want to transfer £100 to a new provider then [subject to particular account restrictions that have nothing to do with the rules for ISAs] you can just ask for that amount and they will 'transfer' it [usually by cheque!] by sending that amount to your new provider.

Of course you are still restricted by silly rules like:

You may not 'subscribe' to more than one ISA in any one tax year... and an 'initial' transfer would constitute an ISA subscription - whereas later transfers would just be 'transfers'.

So for instance, say you have £3000 in ISA 'A' from the year before last, and that you have £3000 in ISA 'B' from last year. You then add £3000 to ISA 'B' [your 'subscription'] at the start of this tax year, and then promptly ask the provider to 'transfer' £1500 of the money in 'B' into 'A' [because you really wanted to pay equal amounts into each account for some perverse reason - and ISA 'B' is instant access]

Now the '£1500' in this example must come from a non current tax year - which it can because £3000 was put in LAST year, and only £1500 of that is being asked for.

My question really is - is this correct information does anyone know, as it comes from the ISA adviser at Nationwide BS H.O. and they are deemed a 'specialist' unit?

Thanks in advance