If it is dual test then its 1st retest can be either DOT or Pi, then any subsequent test must be the same standard as 1st retest
Yes indeedy, that's right.
out of interest what would happen if you were using a DOT bottle in the UK is it more likely to blow up or something?
No you are not likely to blow up, none that I'm aware of have exploded, DOT bottles basically are tested to a higher pressure which it is thought can weaken them. The other camp thinks that just means they are tested more rigorously, and therefore safer.
You can use DOT tanks as long as you buy a dive tank and fill them yourself, but as someone else has said you won't be insured.
On a different note, is there any legislation on this, or is it more because of sites/insurance decisions only?