  We have measured the temperature dependence of the absolute value of the
magnetic penetration depth $\lambda(T)$ in a
Ca$_{10}$(Pt$_{3}$As$_{8}$)[(Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$]$_{5}$ (x=0.097)
single crystal using a low-temperature magnetic force microscope (MFM). We
obtain $\lambda_{ab}$(0)$\approx$1000 nm via extrapolating the data to $T = 0$.
This large $\lambda$ and pronounced anisotropy in this system are responsible
for large thermal fluctuations and the presence of a liquid vortex phase in
this low-temperature superconductor with critical temperature of 11 K,
consistent with the interpretation of the electrical transport data. The
superconducting parameters obtained from $\lambda$ and coherence length $\xi$
place this compound in the extreme type \MakeUppercase{\romannumeral 2} regime.
Meissner responses (via MFM) at different locations across the sample are
similar to each other, indicating good homogeneity of the superconducting state
on a sub-micron scale.
