Thanks Pete –
I gleaned this from the data sheets:
Loctite 5770: After 24 hours @ 25 °C Breakloose Torque, ISO 10964, Pre-torqued to 5 N·m: 3/8 x 16 steel nuts and bolts (grade 5) (degreased) N·m 9.0 to 28.4LMS (lb.in.) (79.6 to 251)
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Loctite 572: After 24 hours @ 22 °C Breakaway Torque, ISO 10964: M10 steel nuts and bolts N·m 7 (lb.in.) (60)
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ROCOL STEAMSEAL is a high pressure, setting pipe sealant for metallic threaded and flanged joints.
Joints sealed with ROCOL STEAMSEAL are easy to break and clean.
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I find this kind of info difficult to interpret, and it seems to me that one can't make a like-for-like comparison between the two Loctites as the parameters are different. However, it *looks* to me that the 572 might be easier to loosen. However, the info that Steamseal is suitable for threaded joints led me in that direction insteaad, so I rang Heritage Steam Supplies in the UK who sell it. They put me on to their tech guy (involved with steam engines all of his life, steam cars and so on) and he advised me against the Steamseal as – he said – it does in fact set pretty hard (because of the manganese in it I think). In fact, he said 'just use PTFE tape – it will work perfectly well'. He was not at all convinced that there is a risk that shreds of PTFE will clog valves and so on.
He did, however, go on to say that if I want to use a sealant, use Wellseal, which does not set hard and is easy to dismantle if required. It can be used as a standalone or in combination with PTFE tape. 'Don't get yourself involved with Loctite and all that stuff', he said.
As a rookie I expected this to be simple but as usual it's tortuous and complex territory, and people have many different opinions. Interesting stuff!
So… given that in the end one has to make a choice, I have ordered a tube of Wellseal. Am not sure about the PTFE tape at this stage – have used it for hydro testing but for steam I might try the Wellseal on its own.
Feel free to comment further, but in any case thanks for all of your input on this.
Will report back in due course 
gary