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Can I transfer money from a personal pension to a SIPP, while leaving the original pension open?
Yes it's entirely possible; see below.  If you can't find anything on transfers out (partial or otherwise) on anyone's site it's because they don't want to give anyone ideas. I have successfully done exactly what you're proposing earlier this year, transferring most of the value from my employer's group personal pension scheme - also Aviva! - to a much lower-cost SIPP.  The lack of any sign of movement by Aviva to post-RDR "clean priced" charge-levels on funds was the final straw for me.  My only regret is that I didn't do it sooner!  Transfer paperwork was initiated from the SIPP end but I was careful to make clear to HR people and Aviva's rep (or whatever group-scheme/employee benefits middleman organization he was from) that I was not exiting the company scheme and expected my employee and matching employer contributions to continue unchanged (and that I'd not be happy if some admin mess up led to me missing a month's contributions). There's a bit more on the affair in a thread here.  Aviva's rep did seem to need a bit of a prod to finally get it to happen.  With hindsight my original hope of an in-specie transfer does seem naive, but the out-of-the-market time was shorter and less scary than anticipated. Just in case you're unaware of it, Monevator's online broker list is an excellent resource to help decide who you might use for a SIPP; cheapest choice depends on level of funds and what you're likely to hold in it and how often you'll trade.