I assume the 1099-R isn't coded "G". If it's coded "G", you don't have to file an amended return.
Assuming it's not a code "G" 1099-R, the answer is Yes. The IRS will notice and fix anything over $50. If you do not amend, they will amend on your behalf and send you a bill. The post being what it is, you might not get the first, second, or even third version of the bill. You could end up with a lien or levy on your remaining US assets. They can and will garnish your pension payments and social security payments. Letting this slide could destroy your US credit rating. Take responsibility and fix your mistake.
As you live here, I assume you are remitting your pension funds here. Thus, you are taxed here on the pension, which means the US-source pension is resourced as UK on Form 1116. When you amend, you will need to also amend 1116. This may be difficult; you may want to hire someone to help you with this.
Warning to all: If you don't get a statement in the post by the date you're expecting it, it doesn't mean it isn't coming or that it hasn't been lost in the post. Always keep track of all income you receive each year, and if you don't get a statement, call the provider and find out what happened to it.
