Hi Lorn250
I got heavily sh*t on in exactly the same way though not by a high street brand name. The objective of getting you over the £25k barrier is that it then becomes an 'unregulated' credit agreement and thus giving you less protection under (i believe) consumer credit laws.
In principle exactly the same thing happened, the PPI was sold to me on the basis that I would get every penny back in 10 years time (this was on a £35k loan over 20 years). However, life changes, and I was in a position to finally get out of life long debt, so 15 months after taking out a £35k loan I asked for a settlement fee, and yep, you guessed it, they put the £10k cost of the insurance onto the loan amount (so you also pay interest on that aswell) and my settlement figure was just under £45k - But whats worse (and believe me, I threw myself down the stairs many times over this, the b***ard insurance was only for 5 years!!!) cleverly put in small writing (and i mean small 1mm high text) that said "Should the loan duration exceed 60 months, your insurance will terminate on the 60th month" This was how they told my that the £10k insurance fee would only cover the first 5 years of a 20 year loan! - Despite numerous visits to the CAB and even to solicitors at my expense, there was no way out for me. so I paid up and swore never to get caught out again.
From your perspective though I would definately say that getting you to agree to the extra £100 (to push you out of the regulated credit agreement envelope) was as good as miss-selling because this makes it easier for them to avoid any fallout from the Consumer Credit Act 1974
Not sure if the following hyperlink will appear on here but it will explain what protection a regulated credit agreement would of given you if you hadn't taken out the extra £100
Consumer Credit Act 1974