Flags cause controversy and sometimes prompt raw and dangerous emotion.

I'm amazed at the anger that arises when the Union flag is hung upside down – it beats me how anyone notices.

Johnny Cash could bring the house down with his emotive rendition of his paean to the US Stars and Stripes – Ragged Old Flag. Sometimes he introduced the song by saying he cherished the freedom Americans had, even the right to burn the national flag. Then he'd add: "But if you burn my flag – I'll shoot you."

Some American patriots are annoyed that Barack Obama does not use the Stars and Stripes flag as the backdrop for his announcements from the White House, as many previous presidents did. Do such things matter? For some people, they matter very much.

I don't take flags and what they symbolise too seriously, but I was moved to anger a couple of years ago, when I was a guest at a wedding held in a country house hotel in a remote part of central Scotland.

When my wife and I went for a walk round the environs before the official proceedings started, we came across a house further up the glen. In its garden the national flag of England, the flag of St George, was flying proudly.

This annoyed me, and also my wife. I thought that later, under cover of darkness, I might organise a raiding party from among the wedding guests. We'd chop down the flagpole, though not necessarily burn the flag. Needless to say, I didn't do this – but I did find the flagrant flying of this flag in the heart of Scotland offensive. If it had been a Union flag, that would have been perfectly acceptable. The St George's Cross, on the other hand, was provocative.

Was whoever flew that flag merely guilty of a little marginal insensitivity? Or was it something more brazen – an act that could be construed as genuinely inflammatory?

When I was a student at an English university, I had a Scottish Saltire on the wall of my room. I was surprised when one of my friends, a normally mild-mannered chap from Nottingham, took offence and suggested it might be better if I took it down. I refused; he was quite miffed.

So I confess: I'm guilty of inconsistency, although there's a difference between a flag in a private room and one flying publicly in the open air. The point is this: flags and offence go together.

Yesterday was St George's Day and many sincere English patriots are appalled that both their national day, and their national flag, the St George's Cross, have been hijacked by some unpleasant groups on the far right. The National Front, which is unfortunately enjoying something of a revival in England, is one of the organisations that likes to flaunt the St George's Cross.

This understandably upsets and confuses many decent folk who are proud of their national flag. But what are they to do? Find another flag which would be a more acceptable symbol of English national pride? After all, St George's links with England were tenuous, to say the least (the same applies to St Andrew and Scotland).

Yet would finding a new flag not just be giving in to thugs, bigots and hard-liners? The dilemma adds considerably to the current confusion surrounding English national identity.

Flags also cause controversy on the high seas, where so-called flags of convenience are cynically but legally used by ship owners to evade the stringent rules of their own countries.

It is accepted practice to register a merchant ship in a foreign country which has a more lax attitude to shipping regulations than your own. The widespread use of flags of convenience, now tolerated globally, started about 80 years ago when some US ship owners registered their vessels in Panama. Now it is estimated that more than half of the world's merchant ships fly such flags.

While the practice is perfectly legitimate, it can still cause controversy and anger. But then, flags and contention go together.