IRS 8833, UK SIPP, which IRC?

I asked this question over at BritishExpats and got the advice (from nun) to post here. This site seems to have a lot of information so here's hoping!

Brief background:
Dual US/UK citizen, originally from UK, left the UK with various private pension plans in place and then, 18 months ago I moved them all into a SIPP which has been doing very well since then. Obviously all these gains are unrealized and no contributions have been made to the SIPP since it was opened.

As far as I can see, since I received no income or distribution from this SIPP, but it did do well and realized some Capital Gains, I only need to file the FBAR/FATCA (Yes it's enough for both of those) and then file the 8833 to invoke article 18 of the tax treaty on the gains.

But where I'm drawing a blank is what to put in the 8833 box "List the Internal Revenue Code provision(s) overruled or modified by the treaty-based position" and also is there anything to put in box 4 on this same form - "List the provision of the limitation on benefits article in the treaty that the taxpayer relies on to prevent application of this article".

Phew, I consider my command of English to be pretty good, who the hell writes this stuff???

I'm aware that the range of professional opinions on SIPP IRS filing goes from "only FBAR and FATCA are required since it's a pension under the language of the treaty" to "you need to file 3520, 3520-A, 8833, FATCA, FBAR and carefully examine any and all investments in the SIPP for PFIC's". I'm trying to steer the middle course here.
I'm sure many will say to use a professional and I understand that POV but even professionals cannot agree on this and my view is that they will always take the "ass protect" view (their ass, not mine). Plus common sense tells me that this should NOT be rocket science - I'm a poor individual simply trying to ensure that my final years not be pennyless - it simply shouldn't be necessary to employ a professional to sort this out.

Hopefully you reported the LSD 18 months ago and have continued to track your basis in the SIPP?

These are complex questions; I empathise with your approach entirely; but you may want to check first with your SIPP trustee in case they want to provide a foreign grantor trust statement to you.

You may also need to report PFICs for years 2012 onwards under the HIRE Act.

I am also frustrated by the way the IRS treats UK pensions, or the lack of certainty at least. The Treaty is supposed to address just these issues to encourage the movement of professionals and trade etc. The UK is pretty good about US pensions, 401ks etc, but the IRS seems to completely ignore any UK retirement account wrapper apart from traditional employer defined benefit plans. If it walks like a retirement plan and quacks like a retirement plan, then it's a retirement plan. There needs to be some IRS rulings and guidance on this rather than competing professional opinions.

Here is an IRS letter mentioning SIPPs. It is pretty non-comital leaving it open as to whether a SIPP qualifies as a pension plan under the treaty, but if it does your Tax Treaty exclusion approach should be ok.

http://www.irs.gov/pub/irs-wd/10-0151.pdf

But you need to be able to prove that your previous pension and the SIPP are pension plans under the Treaty. To do that I'd talk to www.britishamericantax.com.

Here is another discussion about UK pension plans

http://www.taxalmanac.org/index.php/Discussion:US_taxation_of_UK_Stakeholder_Pension_/_SIPP

With the implementation of NEST every US citizen working in the UK will have to deal with a UK DC pension on their US taxes - surely we can get some certainty now!