"...they can't just offer any old salary and benefits package to a prospective employee..."
Not if they're employed as civil servants, no. However HMRC have formed a limited company (RCDTS) to employ the people they're taking from Aspire through the not-quite-TUPE process. This means they can pay whatever they like.
Unfortunately they still have to go to CabO to get everything signed off, so the salaries aren't *that* much better than civil service standard, and the pension terms are pretty crap.
Given that the guys they really want (the people who've been doing this for 15-20 years and know complex+critical systems like PAYE/RTI and VAT inside and out) are all on their third TUPE and have managed to wrangle both good private sector salaries and public sector-esque terms + pensions, the uptake has been unsurprisingly low.
The vast majority of people who've taken the deal are junior staffers who were underpaid by Cap/FJS et al anyway (think sub-£20k). The logical consequence of this is either they're going to have to bring in the experienced bods as time hire contractors at extortionate day rates or they're just going to have to bring in the incumbent suppliers anyway. This is currently being labelled the "co-sourcing" option; same staff, same suppliers, but the paperwork will be shuffled so that HMRC will take delivery responsibility*.
*Judging whether HMRC have the capability to deliver after 20 years with no in-house IT whatsoever is an exercise left to the reader.