Holiday Lettings is a part of the TripAdvisor shared platform (along with FlipKey), which means you will also be required to make an iCal connection to synchronize with their calendars. The method outlined below will allow for two-way synchronization, which eliminates double booking instances.
Step 1: Calendar Exporting
In Sympl: Navigate to the Channels module from the top menu bar.

In the Channels List view, click the + Add Channel button.

In the Channels Overlay, select the iCal tab and locate the Holiday Lettings option. Click Connect or on the + icon if there are existing connections.

In the Exported Calendars section, click the Add icon.

Select the rental from the dropdown list and click Export.

In the popup, copy your Sympl iCal URL.

In Holiday Lettings : Once signed in, locate and click the "Calendar" button at the top of your screen.

Then select the "Import" button from the Calendar page once loaded.
On the Choose a Calendar page, click the "Other" option.

When the "other" calendar page loads, you will be able to paste your exported calendar URL into the URL field and give your calendar import a name. Click the option to connect this calendar in order to complete the process.
Step 2: Calendar Importing
In Holiday Lettings : On the Calendar page, you can locate and click the "Export" button.
The subsequent page will provide you with a calendar URL - copy this to your clipboard and return to your Sympl dashboard.

In Sympl: Navigate to the Channels module from the top menu bar.

In the Channels List view, click the + Add Channel button.

In the Channels Overlay, select the iCal tab and locate the Holiday Lettings option. Click Connect or on the + icon if there are existing connections.

Scroll down to the Imported Calendars section and click the Add icon.

Complete the required information:

What Happens Next?
Sympl will import all of your Holiday Lettings bookings and blocked dates.
All imported Holiday Lettings events become Hold Events in Sympl.
iCal is a calendar link only. You will want to continue managing rates in Holiday Lettings.
You can convert Hold Events to Bookings in Sympl and enter additional info, by clicking on the Hold Event in your Sympl calendar and pressing "Edit", then "Convert".