Recent comments have made me think that "SWT have loads of spare DMUs" is becoming Received Wisdom on here.
They've loaned a DMU to FGW (who need capacity over the summer), just like East Coast have loaned an HST power car to XC, but I don't see people complaining that East Coast have stockpiled too many power cars (or that EMT had too many HSTs, when they surrendered one to East Coast).
But the Lymington shuttle plus the handful of other DMU services a day run wholly on electrified lines (which is partly a scheduling thing - just like Northern ran/run some DMUs wholly under the wires to Ilkley/ Hazel Grove etc - but nobody has ever complained that Northern had too many DMUs).
Maybe it's just me, but I'm noticing that SWT's "surplus" of DMUs is getting mentioned more and more.