....The treaty terms seem very unclear and the various "experts" seem to have conflicting opinions about everything.
I'm also wondering about this form 8333(?) relating to the tax treaty - when do people use it, and would it be necessary in my position?
The need to file 8833 is one of those areas which may be unclear. You mention you do have some defined contribution UK pensions. For the pros, most will have a definite opinion as to filing both 8833 or additional forms; but talk to the IRS and you may receive a different opinion. I'll leave it to others to debate this. IMHO, probably not given what you have described providing you are happy with the results.
And on the 1040, is it sufficient just to put the amount of the pension income on line 16a? Is there any further information required, or a supporting schedule?
The IRS will not expect any verification of the amount to be submitted with your return. Since it's from the UK, the IRS computers will have no understanding of the documents anyway. If a question does arise, then you will need accurate records as to how you arrived at the amount. For the UK State Pension, you may use line 16a and b, or you may use line 21, labelling the amount as
"Foreign Social Security Pension".For the next few years, until I collect my state pension, I don't expect to be paying UK income tax so the tax credit form would not be relevant.
And if I'm earning less than the total of the standard deduction plus personal exemption do I need to file a tax return at all? I'm married to a UK citizen.
If you file as Married Filing Separate (MFS) which is recommended for someone in your situation, the threshold for filing a US tax return (for 2014, for example) is $3,950. Be sure you understand the options of filing Married Filing Jointly (MFJ). The threshold rises to a minimum of $20,300, and can go slightly higher, but you must include all the income of your spouse. If your income, which will include interest from savings accounts (both in your name only or jointly held), dividends, etc., is above the threshold for the year, you should file a US tax return.
The standard deduction is $6,200. For those over 65, it rises to $7,400. The personal exemption is $3,950. (All figures are for 2014.)
Therefore, if you have only pension income plus tax free (ISA) income (or other interest which will fall under the new $1,000 tax free amount), and you fall below the UK Personal Allowance where no UK tax is due, you will have no UK tax paid to offset any US tax due if the total amount of income is above the $10,150 minimum (Std. Deduct plus Pers. Exempt.) for less than 65 years old (or $11,350 if 65+) for the US 1040 return.