Hi Blash
Glad I can be of minor help to at least one person.
Sorry about the delay in replying.
Intermarket relationships - If you mean, for example, always keeping an eye on the dow & S&P if you are trading the FTSE then of course that is always useful information. But this really is only of practical use in the very short term.
You can also watch the interest rate markets, FX markets (Gold, Oil, weather, anything) for clues on what the FTSE may do, but a problem here is 'present relevance'. E.g. The falling pound will be blamed for the FTSE falling so you start to think there is a fast and hard relationship there, everybody is talking about the falling pound and ftse. But just a short time later the pound is still falling but the ftse has stabilised and is even climbing. Now suddenly everybody in the UK equity trading world suddenly seems to have forgotten about the pound and are concentrating on the next 'story'. Happens all the time, this moment rising price of gold is lifting ftse, next moment oil price is pushing ftse down - whatever. So if you are using 'intermarket relationships' in this way you always need to know what is the 'present relevant' story/market.
Actually trading the intermarket relationships e.g FTSE vs Dow (or even more dangerously e.g. FTSE vs Bund) - going short one vs long the other, COULD get dangerous because it leads you into a false sense of security - making you feel that your risk is much reduced when in fact depending on circumstances you could be doubling your risk. This is basically what LTCM did and they near brought the financial market world down when their spread trades went wrong. All their sophisticated statistical models (based on historic data of course - what other kind of data is there?) predicted that what happened would be vitrually impossible, yet sparked by the Russians' defaulting, it did happen.
Some people do v. successfully trade these types of spread, FTSE vs DOW, Bund vs Gilt etc but it takes a slightly different mind set than trading the FTSE/dow/bund outright. Each spread MUST be thought of as a position in it's own right and should be charted/analysed as such. (i've never been able to do it very successfully myself - prefer to be long/short one or the other).
Finally in practical terms there is the added transaction costs of trading these spreads i.e. doubling of commission and two lots of bid-offer spread.
Hope this has answered you question
regards